Wireless Resonant Circuit and Variable Inductance Vascular Monitoring Implants and Anchoring Structures Therefore

ABSTRACT

Wireless, variable inductance and resonant circuit-based vascular monitoring devices, systems, methodologies, and techniques, including specifically configured anchoring structures for same, are disclosed that can be used to assist healthcare professionals in predicting, preventing, and diagnosing various heart-related and other health conditions.

RELATED APPLICATION DATA

This application is a continuation of PCT/US2019/034657, filed May 30,2019, which international application claims the benefit of priority ofU.S. Provisional Patent Application Ser. No. 62/678,237, filed May 30,2018, and titled “Wireless Resonant Circuit and Variable InductanceVascular Monitoring Implants and Anchoring Structures Therefore”. Thisapplication is also a continuation-in-part of U.S. Nonprovisional patentapplication Ser. No. 16/177,183, filed on Oct. 31, 2018, and titled“Wireless Vascular Monitoring Implants”, which application was a 371application of PCT/US17/63749, filed Nov. 29, 2017, and whichinternational application claims the benefit of priority of U.S.Provisional Application No. 62/534,329 filed Jul. 19, 2017, and U.S.Provisional Application No. 62/427,631, filed Nov. 29, 2016. Each ofthese applications is incorporated by reference herein in its entirety.

FIELD OF THE DISCLOSURE

The present invention generally relates to the field of vascularmonitoring. In particular, the present invention is directed to wirelessvascular monitoring implants, systems, methods, software and anchoringstructures therefore. More specifically, embodiments disclosed hereinrelate to fluid volume sensing in the inferior vena cava (IVC) usingwireless, remotely or automatically actuatable implants for monitoringor management of blood volume.

BACKGROUND

Others have attempted to develop vascular monitoring devices andtechniques, including those directed at monitoring vessel arterial orvenous pressure or vessel lumen dimensions. However, many such existingsystems are catheter based (not wireless) and thus can only be utilizedin a clinical setting for limited periods of times, and may carry risksassociated with extended catheterization. For a wireless solution, thecomplexity of deployment, fixation and the interrelationship of thosefactors with detection and communication have led to, at best,inconsistent results with such previously developed devices andtechniques.

Existing wireless systems focus on pressure measurements, which in theIVC can be less responsive to patient fluid state than IVC dimensionmeasurements. However, systems designed to measure vessel dimensionsalso have a number of drawbacks with respect to monitoring in the IVC.Electrical impedance-based systems require electrodes that arespecifically placed in opposition across the width of the vessel. Suchdevices present special difficulties when attempting to monitor IVCdimensions due to the fact that the IVC does not expand and contractsymmetrically as do most other vessels where monitoring may be desired.Precise positioning of such position-dependent sensors is a problem thathas not yet been adequately addressed. IVC monitoring presents a furtherchallenge arising from the physiology of the IVC. The IVC wall isrelatively compliant compared to other vessels and thus can be moreeasily distorted by forces applied by implants to maintain theirposition within the vessel. Thus devices that may perform satisfactorilyin other vessels may not necessarily be capable of precise monitoring inthe IVC due to distortions created by force of the implant acting on theIVC wall. As such, new developments in this field are desirable in orderto provide doctors and patients with reliable and affordable wirelessvascular monitoring implementation, particularly in the critical area ofheart failure monitoring.

SUMMARY OF THE DISCLOSURE

Embodiments disclosed herein comprise wireless vascular monitoringdevices, circuits, methodologies, and related techniques for use inassisting healthcare professionals in predicting, preventing, anddiagnosing various conditions whose indicators may include vascularfluid status. Using embodiments disclosed, metrics including, forexample, relative fluid status, fluid responsiveness, fluid tolerance,or heart rate may be accurately estimated.

In one implementation, the present disclosure is directed to animplantable vessel monitoring device configured and dimensioned to beimplanted in a patient blood vessel in contact with the vessel wall. Thedevice includes an expandable and collapsible variable inductance coilcomprising a plurality of adjacent wire strands formed around an opencenter to allow substantially unimpeded blood flow therethrough, thecoil configured and dimensioned (i) to extend around an inner peripheryof the vessel when implanted therein and (ii) to move with the vesselwall in response to expansion and collapse of the vessel; and acapacitance which together with the variable inductance coil forms avariable inductance resonant circuit having a variable characteristicfrequency correlated to the diameter or area of the expandable andcollapsible variable inductance coil

In another implementation, the present disclosure is directed to animplantable vessel monitoring device configured and dimensioned to beimplanted in a patient blood vessel in contact with the blood vesselwall. The device includes an expandable and collapsible variableinductance coil comprising at least about 150 adjacent wire strandsformed around an open center, the coil configured and dimensioned (i) toextend around an inner periphery of the blood vessel when implantedtherein and (ii) to move with the blood vessel wall in response toexpansion and collapse of the blood vessel; a sensor frame formed in asingle loop having a loop circumference on which the variable inductancecoil is supported, the plurality of wire strands forming a wire coatingover the sensor frame and being wrapped in at least one loop around theloop circumference of the sensor frame; and a capacitance which togetherwith the variable inductance coil forms a variable inductance resonantcircuit having a variable characteristic frequency correlated to thediameter or area of the expandable and collapsible variable inductancecoil.

In still another implementation, the present disclosure is directed toan implantable vessel monitoring device configured and dimensioned to beimplanted in a patient vena cava in contact with the vena cava wall. Thedevice includes an expandable and collapsible variable inductance coilcomprising a plurality of substantially parallel wire strands formedaround an open center, the coil configured and dimensioned (i) tosurround an inner periphery of the vena cava when implanted therein and(ii) to move with the vena cava wall in response to changes in fluidvolume or movement of the vena cava wall over the respiratory andcardiac cycles; a flexible sensor frame supporting the variableinductance coil and being configured and dimensioned (i) to maintain thevariable inductance coil in contact with the vena cava wall, (ii) tomove with expansion and collapse of the vena cava wall over therespiratory and cardiac cycles, and (iii) to allow changes in shape ofthe variable inductance coil corresponding to a magnitude of collapseand expansion as the vena cava varies with changes in vascular fluidvolume of the patient; a capacitance which together with the variableinductance coil forms a variable inductance resonant circuit having avariable characteristic frequency correlated to the diameter or area ofthe expandable and collapsible variable inductance coil; and at leastone device anchor attached to the resilient loop frame by at least oneanchor isolation section, the anchor isolation section being configuredto allow relative motion between the at least one device anchor and theresilient loop frame.

In still another implementation, the present disclosure is directed toan implantable vessel monitoring device configured and dimensioned to beimplanted in a patient vena cava in contact with the vena cava wall. Thedevice includes an expandable and collapsible variable inductance coilcomprising at least about 150 adjacent wire strands formed around anopen center to allow substantially unimpeded blood flow therethrough,the coil configured and dimensioned (i) to extend around an innerperiphery of the vena cava when implanted therein and (ii) to move withthe vena cava wall in response to changes in fluid volume or movement ofthe vena cava wall over the respiratory and cardiac cycles; a sensorframe formed in a single loop having a loop circumference with pluralsubstantially straight sections connected by bends to form a resilient,open center, zig-zag shaped loop structure supporting the expandable andcollapsible variable inductance coil with the plurality of wire strandsbeing wrapped in at least one loop around the loop circumference of thesensor frame to form a wire coating covering the sensor frame; an outerinsulating layer covering the plurality of wires and sensor frame; acapacitance which together with the variable inductance coil forms avariable inductance resonant circuit having a variable characteristicfrequency correlated to the diameter or area of the expandable andcollapsible variable inductance coil; a zig-zag loop anchor frame formedby plural anchor sections joined by crown sections, with at least oneanchor barb disposed in each the anchor section; and at least one anchorisolation section concentrically joining the zig-zag loop anchor framewith the zig-zag loop sensor frame, the anchor isolation section beingconfigured to allow relative motion between the sensor frame and theanchor frame.

These and other aspects and features of non-limiting embodiments of thepresent disclosure will become apparent to those skilled in the art uponreview of the following description of specific non-limiting embodimentsof the invention in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the disclosure, the drawings showaspects of one or more embodiments of the disclosure. However, it shouldbe understood that the present disclosure is not limited to the precisearrangements and instrumentalities shown in the drawings, wherein:

FIG. 1 schematically depicts an embodiment of a wireless resonantcircuit-based vascular monitoring (“RC-WVM”) system of the presentdisclosure;

FIG. 1A schematically depicts a portion of an alternative embodiment ofa RC-WVM system of the present disclosure;

FIGS. 2 and 2A illustrate alternative embodiments of RC-WVM implantsmade in accordance with the teachings of the present disclosure;

FIG. 2B is a schematic, detailed view of the capacitor section of theRC-WVM implant illustrated in

FIG. 2;

FIGS. 3, 3A, 3B, 3C and 3D illustrate an embodiment of a belt antenna asdepicted schematically in the system of FIG. 1;

FIG. 3E schematically depicts the orientation of the antenna belt andmagnetic field generated thereby with respect to an implanted RC-WVMimplant;

FIG. 4 is a block diagram illustrating an embodiment of systemelectronics;

FIGS. 5A and 5B illustrate fixed frequency RF burst excitation signalwave forms;

FIGS. 6A and 6B illustrate sweep frequency RF burst excitation signalwave forms;

FIG. 7 is a block diagram depicting a multi-channel, direct digitalsynthesizer used in signal generation modules of control systems inembodiments disclosed herein;

FIGS. 7A and 7B illustrate multi-frequency RF burst excitation signalwave forms;

FIG. 8 illustrates waveform pulse shaping;

FIGS. 9A, 9B, 9C and 9D schematically illustrate aspects of anembodiment of a delivery system for RC-WVM implants as disclosed herein,wherein FIG. 9A shows an overall view of the delivery system and itssub-components, FIG. 9B shows a detail of the distal end with the RC-WVMloaded, FIG. 9C depicts a partial deployment of an RC-WVM implant fromthe delivery sheath into the IVC, and FIG. 9D shows the distal end of analternative embodiment of a delivery system for an alternative RC-WVMimplant with an attached anchor frame as disclosed herein;

FIGS. 10A, 10B, 10C, 10D and 10E illustrate signals obtained inpre-clinical experiments using a prototype system and an RC-WVM implantas shown in FIGS. 1 and 2;

FIGS. 11A and 11B schematically depict components and possiblearrangements of alternative clinical or home systems employing RC-WVMimplants and control systems as disclosed herein;

FIGS. 12A, 12B, 12C, 13A, 13B, 13C, 13D, 14A, 14B, 15A, 15B, 16A, 16B,17A, 17B, 18, 19A, and 19B illustrate alternative embodiments of RC-WVMimplants according to the present disclosure;

FIGS. 20A and 20B illustrate alternative frame structures for use in anRC-WVM implant as disclosed herein;

FIGS. 21A and 21B illustrate an example of a method of making an RC-WVMimplant embodiment according to the present disclosure;

FIG. 22A illustrates an alternative system in accordance with thepresent disclosure for energizing and communicating with RC-WVMimplants, including a planar antenna module with send and receive coils;

FIG. 22B schematically depicts a further alternative antenna module;

FIGS. 23A and 23B illustrate signals obtained in pre-clinicalexperiments using the prototype implant shown in FIG. 12A and antennamodule configuration shown in FIG. 22B;

FIG. 24A is a circuit diagram of an example excitation and feedbackmonitoring (“EFM”) circuit that can be used with embodiments of RC-WVMimplants and systems as described herein;

FIG. 24B is a circuit diagram of another example EFM circuit that can beused with embodiments of RC-WVM implants and systems as describedherein;

FIG. 25A is a circuit diagram of an antenna module tuning and detuningnetwork that can be used with an EFM circuit like that of FIG. 24A or24B;

FIG. 25B schematically depicts a further embodiment of antenna modulecoils arranged to provide geometric decoupling of the transmit andreceive signals;

FIG. 26A illustrates an alternative signal generation module for systemsaccording to embodiments disclosed herein;

FIG. 26B illustrates an alternative receiver chain signal conditioningmodule for use in systems according to embodiments disclosed herein;

FIG. 26C illustrates an alternative data conversion module for use insystems according to embodiments disclosed herein;

FIGS. 27A and 27B illustrate alternative belt antenna embodimentsutilizing variable length of coil features;

FIGS. 28A and 28B illustrate alternative active and passive diodeswitches for use in antenna element embodiments disclosed herein;

FIGS. 29A and 29B illustrate alternative antenna belt embodiments;

FIGS. 30A and 30B are block diagrams illustrating alternative controlsystems with an on-board, implanted, power supply;

FIGS. 31A and 31B are perspective views of alternative embodiments ofwireless implants with an on-board power supply and control electronicsaccording to further embodiments disclosed herein;

FIG. 32 is a schematic depiction of a wireless implant includingon-board power and electronics communicating with an implanted cardiacmonitoring device; and

FIG. 33 is a block diagram depicting one possible embodiment of acomputer-based implementation of aspects of an exemplary control systemin the form of a specialized computing device or system.

FIGS. 34A, 34B and 34C illustrate a further alternative RC-WVM implantembodiment in accordance with the teachings of the present disclosure;

FIG. 35 illustrates assembly of an alternative RC-WVM implant embodimentsuch as shown in FIGS. 34A-C;

FIG. 36 is a detailed view of an anchor structure mounted on an implantprior to encapsulation;

FIGS. 37A, 37B and 37C illustrate an alternative anchor structure foruse with RC-WVM implant embodiments;

FIGS. 38A and 38B illustrate an alternative embodiment of a belt antennafor use with RC-WVM implants and systems as described herein;

FIGS. 39A and 39B illustrate recapture features to facilitatepositioning and repositioning of RC-WVM implants during placement usinga delivery catheter as disclosed herein;

FIG. 40 is a perspective view of an alternative RC-WVM implantembodiment with an attached anchor frame and axial anchor barbs;

FIG. 41 is a perspective view of an anchor frame as shown, for examplein FIG. 40;

FIG. 42 is a detail view showing attachment of an anchor frame to astrut section of a RC-WVM implant;

FIG. 43 is a detail view showing a split in the anchor frame to preventmagnetic field coupling with the anchor frame;

FIG. 44 illustrates a further alternative embodiment in which anchorframes are disposed on both ends of a RC-WVM implant;

FIGS. 45A, 45B and 45C illustrate another embodiment of an anchor framewith anchor barbs oriented parallel to the anchor frame struts;

FIGS. 46A. 46B and 46C illustrate another embodiment of an anchor framewith anchor barbs oriented in the direction of flow in the vessel inwhich the RC-WVM is implanted;

FIGS. 47A and 47B illustrate yet another embodiment of an anchor framewith anchor barbs positioned at the crowns of the anchor frames;

FIG. 48A illustrates a shape set anchor frame with adjacent anchor barbson the same side of the frame strut, and FIG. 48B shows an alternativewith double anchors at each anchor location;

FIGS. 49A, 49B, 49C, 49D, 49E, 49F, 49G and 49H each illustratealternative embodiments of anchor barbs;

FIG. 50 is a schematic cross-section showing a non-conducting connectionof two anchor frame parts;

FIG. 51 shows a perspective view of a further alternative anchor frameembodiment; and

FIGS. 52A, 52B, 52C and 52D each show different alternative embodimentsof anchor frame attachment arms.

DETAILED DESCRIPTION

Aspects of the present disclosure are directed to wireless, resonantcircuit-based vascular monitoring (“RC-WVM”) implants, systems, methods,and software, including excitation and feedback monitoring (“EFM”)circuits that can be used to energize an RC-WVM implant with anexcitation signal and receive characteristic feedback signals producedby the RC-WVM implant. By automatically or manually analyzing thefeedback produced by the RC-WVM implant, it is possible to assisthealthcare professionals in predicting, preventing, and diagnosingvarious heart-related, kidney-related, or vascular-related healthconditions. For example, the feedback produced by the RC-WVM implant ata particular time can be compared to feedback produced by the RC-WVMimplant at other times and/or feedback produced by a baseline RC-WVMimplant in order to understand vessel geometry and therefore estimaterelative fluid status, fluid responsiveness, fluid tolerance, heartrate, respiration rate and/or other metrics. One or more of theseestimations can be generated automatically or manually in order tomonitor the status of a patient and provide feedback to a healthcareprofessional and/or the patient in case of any anomalies or relevanttrends.

System Overview

The unique physiology of the IVC presents some distinctive challenges inattempting to detect and interpret changes in its dimensions arisingfrom changes in patient fluid state. For example, the IVC wall in atypical monitoring region (i.e., between the hepatic and renal veins) isrelatively compliant compared to other vessels, which means that changesin vessel volume can result in different relative distance changesbetween the anterior-posterior walls as compared to the lateral-medialwalls. Thus, it is quite typical that changes in fluid volume will leadto paradoxical changes in the geometry and motion of the vessel; thatis, as the blood volume reduces the IVC tends to get smaller andcollapses with respiration, and as the blood volume increases the IVCtends to get larger and the collapse with respiration is reduced.Systems and implants disclosed herein are uniquely configured tocompensate for and interpret such paradoxical changes.

As shown in FIG. 1, systems 10 according to the present disclosure maygenerally comprise RC-WVM implant 12 configured for placement in apatient's IVC, control system 14, antenna module 16 and one or moreremote systems 18 such as processing systems, user interface/displays,data storage, etc., communicating with the control and communicationsmodules through one or more data links 26, which may be wired orremote/wireless data links. In many implementations, remote system 18may comprise a computing device and user interface, such as a laptop,tablet or smart phone, which serves as an external interface device.

RC-WVM implants 12 generally comprise a variable inductance, constantcapacitance, resonant L-C circuit formed as a resiliently collapsiblecoil structure, which, when positioned at a monitoring position withinthe patient's IVC, moves with the IVC wall as it expands and contractsdue to changes in fluid volume. The variable inductance is provided bythe coil structure of the implant such that the inductance changes whenthe dimensions of the coil change with the IVC wall movement. Thecapacitive element of the circuit may be provided by a discretecapacitor or specifically designed inherent capacitance of the implantstructure itself. Embodiments of RC-WVM implant 12 also may be providedwith anchoring and isolation means inherently designed into the implantstructure, or with distinct additional such structures, to ensure thatthe implant is securely and properly positioned in the IVC withoutunduly distorting the vessel wall so as to distort or otherwisenegatively impact measurements determined by the implant. In general,RC-WVM implants 12 are configured to at least substantially permanentlyimplant themselves in the vascular lumen wall where placed upondeployment and do not require a physical connection (for communications,power or otherwise) to devices outside the patient's body afterimplantation. “Substantially permanently implanted” as used herein meansthat in normal usage the implant will, throughout its useful,operational life, remain implanted in the vascular lumen wall and may tovarying degrees become integrated into the vascular lumen wall by tissueingrowth, but the implant may be intentionally removed as medicallydictated by an intravascular interventional or surgical removalprocedure specifically undertaken for the purpose of removing theimplant. Details of alternative embodiments of implant 12, shown inFIGS. 2, 2A, 12A, 12B, 12C 13A, 13B, 13C, 13D, 14A, 14B, 15A, 15B, 16A,16B, 17A, 17B, 18, 19A, 19B, and FIGS. 34A-C are provided below. Inparticular, it should be noted that any of alternative RC-WVM implantsdescribed herein may be utilized in alternative systems 10 as describedherein without further modification of the systems except as may beidentified.

Control system 14 comprises, for example, functional modules for signalgeneration, signal processing and power supply (generally comprising theEFM circuits and indicated as module 20) and communications module 22 tofacilitate communication and data transfer to various remote systems 18through data links 26 and optionally other local or cloud-based networks28. Details of alternative embodiments of control system 14, modules 20and 22, and elements of alternative EFM circuits are described below andillustrated in FIGS. 4, 7, 24A, 24B, 25A, 25B, 26A, 26B and 26C. Afteranalyzing signals received from RC-WVM implant 12 after being excited bya transmit coil of an EFM circuit, results may be communicated manuallyor automatically through remote system 18 to the patient, a caregiver, amedical professional, a health insurance company, and/or any otherdesired and authorized parties in any suitable fashion (e.g., verbally,by printing out a report, by sending a text message or e-mail, orotherwise).

Antenna module 16 is connected to control system 14 by power andcommunication link 24, which may be a wired or wireless connection.Antenna module 16 creates an appropriately shaped and oriented magneticfield around RC-WVM implant 12 based on signals provided by the EFMcircuitry of control system 14. The magnetic field energizes the L-Ccircuit of RC-WVM implant 12 causing it to produce a “ring-back” signalindicative of its inductance value at that moment. Because theinductance value is dependent on the geometry of the implant, whichchanges as mentioned above based on dimensional changes of the IVC inresponse to fluid state heart rate etc., the ring-back signal can beinterpreted by control system 14 to provide information as to the IVCgeometry and therefore fluid state. Antenna module 16 thus also providesa receive function/antenna as well as a transmit function/antenna. Insome embodiments the transmit and receive functionality are performed bya single antenna, in others each function is performed by a separateantenna. Antenna module 16 is schematically depicted in FIG. 1 as anantenna belt, which embodiment is described in more detail below andshown in FIGS. 3A-D.

FIG. 1A illustrates one alternative embodiment of antenna module 16 asantenna pad 16 a, in which transmit coil 32 and receive coil 34 aredisposed in a pad or mattress 36 on which the patient lays on his/herback with RC-WVM implant 12 (implanted in the IVC) positioned over coils32, 34. Antenna module 16 as shown in FIG. 1A is functionally equivalentto other alternative antenna modules disclosed herein; it is connectedto control system 14 by power and communications link 24 as describedabove. Further alternative embodiments and components of antenna module16 are also shown in FIGS. 22A, 22B, 27A, 27B, 28A, 28B, 29A and 29B anddescribed in more detail below. Another alternative embodiment of a beltantenna module is shown in FIGS. 15A and 15B. Planar-type antennamodules also may be configured in wearable configurations, e.g., whereinthe antenna coil is integrated into a wearable garment such as abackpack or vest. Antenna module 16 may also comprise a coil adapted tobe fastened directly to the patient's skin by tape, glue or other means,e.g. over the abdomen or back, or integrated into furniture such as achair back. As will be appreciated by persons skilled in the art, thevarious embodiments of antenna module 16 as described herein may beemployed with system 10 as shown in FIG. 1 without further changes tothe system or antenna module other than as specifically identifiedherein.

The variable inductance L-C circuit produces a resonant frequency thatvaries as the inductance is varied. With the implant securely fixed at aknown monitoring position in the IVC, changes in geometry or dimensionof the IVC cause a change in configuration of the variable inductor,which in turn cause changes in the resonant frequency of the circuit.These changes in the resonant frequency can be correlated to changes inthe vessel geometry or dimension by the RC-WVM control and communicationsystem. Thus, not only should the implant be securely positioned at amonitoring position, but also, at least a variable coil/inductor portionof the implant should have a predetermined resilience and geometry.Thus, in general, the variable inductor is specifically configured tochange shape and inductance in proportion to a change in the vesselgeometry. In some embodiments, an anchoring and isolation means willcomprise appropriately selected and configured shape and compliance inthe sensor coil structure of the implant so as to move with the vesselwall while maintaining position. Such embodiments may or may not includeadditional anchoring features as discussed in more detail below.Alternatively, an anchoring and isolation means may comprise a separatestructure spaced and/or mechanically isolated from a variable inductorcoil structure such that the anchoring function is physically and/orfunctionally separated from the measuring/monitoring function such thatany distortion or constraint on the vessel caused by the anchor issufficiently distant and/or isolated from the variable inductor so asnot to unduly affect measurements.

RC-WVM implant 12 as a variable inductor is configured to be remotelyenergized by an electric field delivered by one or more transmit coilswithin the antenna module positioned external to the patient. Whenenergized, the L-C circuit produces a resonant frequency which is thendetected by one or more receive coils of the antenna module. Because theresonant frequency is dependent upon the inductance of the variableinductor, changes in geometry or dimension of the inductor caused bychanges in geometry or dimension of the vessel wall cause changes in theresonant frequency. The detected resonant frequency is then analyzed bythe RC-WVM control and communication system to determine the change inthe vessel geometry or dimension. Information derived from the detectedresonant frequency is processed by various signal processing techniquesas described herein and may be transmitted to various remote devicessuch as a healthcare provider system or patient system to providestatus, or in appropriate instances, alerts or modifications intreatment. In order to facilitate measurement of the detected resonantfrequency, it may be desirable to provide designs with a relativelyhigher Q factor, i.e. resonant circuit configurations that maintainsignal/energy for relatively longer periods, especially when operatingat lower frequencies. For example, to realize advantages of designsemploying Litz wire as further described herein, it may be desirable tooperate in a resonant frequency range of below 5 MHz, typically betweenabout 1 MHz and 3 MHz, in which case resonant circuit configuration witha Q factor of at least about 50 or greater may be desired.

An Example of a Complete System Embodiment

Details of one possible embodiment of a complete, exemplary system 10are discussed hereinafter with reference to FIGS. 2-8. Thereafter,details of further alternative embodiments of system components aredescribed. However, it is to be understood that the exemplary system isnot limited to use of the specific elements or components shown in FIGS.1-9C and that any alternative component thereafter described may besubstituted without change in the overall system except as may be noted.For example, RC-WVM implant 12 or any of alternative RC-WVM implants 12c-k, m, n and p may be substituted for implants 12 a or 12 b as firstdescribed below. Similarly, control system 14 may be provided as shownin any of FIGS. 4, 24A, 24B, 26A, 26B, 26C, 28A, 28B, 29A and 29B and/orantenna module 16 may be provided, for example, as a pad or belt anantenna such as pad antenna 16 a, with a single switched antenna coil orseparate, decoupled transmit and receive coils, or belt antennas 16 b,16 c, 16 d, 16 e or 16 f.

FIG. 2 illustrates one example of RC-WVM implant 12 according to thepresent disclosure as may be used in exemplary system 10. The enlargeddetail in the box of FIG. 2 represents a cross-sectional view taken asindicated. (Note that in the cross-sectional view, individual ends ofthe very fine wires may not be distinctly visible due to their verysmall size). In general, RC-WVM implants 12 comprise a resilient sensorconstruct generally including an inductive coil formed around an opencenter to allow substantially unimpeded blood flow there through,wherein the inductive coil changes inductance with changes in theconstruct geometry as a result of forces applied to it. In this example,implant 12 a is formed as a resilient, concentric zig-zag or linked“Z-shapes” structure with a series of strut sections 38 joined at theirends by rounded crown sections 40 forming acute angles. The resultantstructure may also be considered to be sinusoidal in appearance. Thisstructure may be formed by wrapping conductive wires 42 onto a frame orcore 44. In this alternative, RC-WVM implant 12 a has a shape set 0.010″nitinol wire frame 44 around which 300 strands of 0.04 mm diameter gold,individually insulated, Litz wire 42 are wrapped in a single loop. Witha single loop wrap, the strands of wire 42 appear substantially parallelto the frame at any given point, as can be seen in the cross-sectionalview of FIG. 2. Individual insulation on Litz wires 42 may be formed asa biocompatible polyurethane coating. Also in this particular example,discrete capacitor 46 is provided with a capacitance of approximately47ηF (nano-Farads); however, the capacitance may be in the range ofabout 180 pico-Farads to about 10 micro-Farads, to cover all potentialallowable frequency bands (from about 148.5 kHz to about 37.5 MHz) forRC-WVM implants 12. In one alternative, rather than a relatively largenumber of wire strands in a single loop, a relatively few number ofstrands, e.g. in the range of about 10-20 strands, or more particularlyabout 15 strands, may be arranged in a relatively larger number ofloops, e.g. in the range of about 15-25 loops, or more particularlyabout 20 loops. In this alternative embodiment the discrete capacitorelement is replaced with an inherent coil capacitance that arises basedon spaces between the parallel strands of wire.

In a further alternative embodiment, implant 12 a is configured toensure strut sections 38 are straight strut sections between crownsections 40. Straight strut sections can provide an advantage of thestrut section always being in contact with the vessel wall over itsentire length, irrespective of the size of vessel into which it isdeployed. When the sensor construct frame is formed, for example, bylaser cutting the construct from a nitinol tube, the straightconfiguration of the straight strut sections can be achieved byshape-setting the strut sections to maintain the desired straightconfiguration.

With reference also to FIG. 2B, Litz wire 42 is formed around a shapeset nitinol frame 44. The two ends of Litz wire 42, which may be coveredwith a layer of PET heat shrink tubing 60, are joined together with acapacitor 46 to form a loop circuit. Capacitor 46 includes capacitorterminals 52 connected to Litz wires 42 by solder connection 54 to goldwire contacts 56. Gold wire contacts 56 are formed by removing (orburning away) the individual insulation from a short section at the endof Litz wires 42 and joining those ends to form solid contacts, whichthen may be joined to capacitor terminals 52 by solder connections 54.The capacitor, capacitor terminals and gold wire contacts areencapsulated in an appropriate biocompatible insulating material 58 suchas a reflowed polymer or epoxy. In alternative embodiments, the entirestructure may then be covered by a layer of PET heat shrink insulation60. Alternatively, if determined that a short circuit through the frameshould not be created, a gap may be provided in the frame at thecapacitor or elsewhere.

As shown in FIG. 2, RC-WVM implant 12 a is also optionally provided withanchors 48 to help prevent migration of the implant after placement inthe IVC. Anchors 48 also may be formed of nitinol laser cut sections orshape set wire and bonded to each strut section 38. Barbs 50 extendoutwardly at the end of anchors 48 to engage the IVC wall. In oneembodiment, anchors 48 are bi-directional in both the cranial and caudaldirections; in other embodiments the anchors may be in one direction, amixture of both directions or perpendicular to the vessel.

The overall structure of RC-WVM implants 12 presents a balance ofelectrical and mechanical requirements. For example, an ideal electricalsensor is as close to a solenoid as possible with strut lengths as shortas possible and ideally zero, whereas mechanical considerations ofdeployment and stability dictate that implant strut lengths be at leastas long as the diameter of the vessel into which it is to be deployed toavoid deployment in the wrong orientation and maintain stability.Dimensions of elements of RC-WVM implant 12 a are identified by lettersA-F in FIG. 2, and examples of typical values for those dimensions,suited for a range of patient anatomies, are provided below in Table I.In general, based on the teachings herein, persons skilled in the artwill recognize that the uncompressed, free-state (overall) diameter ofRC-WVM implants 12 should not significantly exceed the largestanticipated fully extended IVC diameter for the patient in which theRC-WVM implant is to be used. RC-WVM implant height generally should beselected to balance implant stability at the monitoring position withgeometry/flexibility/resilience providing the ability to fit in theintended region of the IVC without impacting either the hepatic or renalveins in the majority of the population, which could compromise sensingdata produced by the implant. Height and stability considerations willbe influenced, among other factors, by specific RC-WVM implant designconfiguration and whether or not distinct anchor features are included.Thus, as will be appreciated by persons skilled in the art, primarydesign considerations for RC-WVM implants 12 according to the presentdisclosure are provision of structures forming variable inductance L-Ccircuits with the ability to perform the measuring or monitoringfunction described herein, and which are configured to securely anchorthe structures within the IVC without distortion of the IVC wall byproviding adequate but relatively low radial force against the IVC wall.

TABLE I RC-WVM Implant 12a & 12b Example Dimensions Element ApproximateSize Dim. Name (in millimeters) A Height 10-100, typically about 20 BStrut length 10-100, typically about 25 C Strut diam. 0.1-2, typicallyabout 1.5 F Anchor Length 1-10, typically (extending) about 5 E AnchorLength 0.25-3, typically (barb) about 1.8 D Overall Three Sizes:Diameter 20 mm/25 mm/32 mm +/− 3 mm

Another alternative structure for RC-WVM implant 12 is illustrated byRC-WVM implant 12 b as shown in FIG. 2A. Once again, the enlarged detailin the box of FIG. 2A represents a cross-sectional view taken asindicated. In this embodiment, implant 12 b has an overall structurethat is similar to that of implant 12 a, formed on a frame with straightstrut sections 38 and curved crown sections 40. In this embodiment, thediscrete capacitor for the previous embodiment is replaced withdistributed capacitance between the bundles of strands of wire. Multiple(for example, approximately fifteen) strands of wire 64 are laidparallel to each other and twisted into a bundle. This bundle is thenwrapped, multiple times, around the entire circumference of wire frame66 (which may be, for example, a 0.010″ diameter nitinol wire) resultingin multiple turns of parallel bundles of strands. The insulation betweenthe bundles results in a distributed capacitance that causes the RC-WVMto resonate as previously. Overall dimensions are similar and may beapproximated as shown in Table I. An outer, insulation layer or coating60 may be applied either as previously described or using a dipping orspraying process In this case, the L-C circuit is created without adiscrete capacitor, but instead by tuning the inherent capacitance ofthe structure through selection of materials and length/configuration ofthe wire strands. In this case, 20 turns of 15 strands of wire are usedalong with an outer insulation layer 60 of silicone to achieve acapacitance inherent in implant 12 b in the range of approximately 40-50if.

Unlike implant 12 a, frame 66 of implant 12 b is non-continuous so as tonot complete an electrical loop within the implant as this wouldnegatively impact the performance. Any overlapping ends of frame 66 areseparated with an insulating material such as heat shrink tubing, aninsulating epoxy or reflowed polymer. RC-WVM implant 12 b (may or) maynot include anchors. Instead, the implant is configured to have acompliance/resilience to permit it to move with changes in the IVC wallgeometry or dimension while maintaining its position with minimaldistortion of the natural movement of the IVC wall. This configurationcan be achieved by appropriate selection of materials, surface featuresand dimensions. For example, the strut section length of the frame mustbalance considerations of electrical performance versus stability,wherein shorter strut section length may tend to improve electricalperformance but longer strut section length may increase stability.

In order to energize RC-WVM implant 12 and receive the signal back fromthe implant, antenna module 16 will functionally include a transmit anda receive antenna (or multiple antennas). Antenna module 16 thus may beprovided with physically distinct transmit and receive antennas, or, asin the presently described exemplary system 10, provided by a singleantenna that is switched between transmit and receive modes. Antennabelt 16 b, shown FIGS. 3 and 3A-D, illustrates an example of antennamodule 16 employing a single, switched antenna. A single loop antenna isformed from a single wire and placed around the patient's abdomen. Thiswire antenna is connected directly to the control system 14.

In terms of mechanical construction, antenna belt 16 b generallycomprises stretchable web section 72 and buckle 74 with a connection forpower and data link 24. In one embodiment, in order for the size of theantenna belt 16 b to accommodate patients of different girths (e.g., apatient girth range of about 700-1200 cm), a multi-layer constructionmade up of a combination of high-stretch and low-stretch materials maybe employed. In such an embodiment, base layer 76 is a combination ofhigh-stretch sections 76 a and low-stretch section 76 b, which arejoined such as by stitching. Outer layer 78, with substantially the sameprofile as base layer 76, may be comprised entirely of the high-stretchmaterial, which may be a 3D mesh fabric. Within each section, antennacore wire 82 is provided in a serpentine configuration with an overalllength sufficient to accommodate the total stretch of the section. Corewire 82 should not itself stretch. Thus, the stretchability of thefabric layers is paired with the core wire total length to meet thedesired girth accomodation for a particular belt design. Outer layer 78is joined along the edges to base layer 76. Stitching covered by bindingmaterial 80 is one suitable means for joining the two layers. The layersmay be further bonded together by a heat fusible bonding material placedbetween the layers. End portions 81 of web section 72 are configured forattachment to buckle 74.

Core wire 82, which forms the antenna element, is disposed between thelayers and provided with an extendable, serpentine configuration so thatit may expand and contract with the stretch of the belt. A mid-section84 of core wire 82, which corresponds to low-stretch section 76 b, has agreater width. This section, intended to be placed in the middle of thepatient's back with antenna belt 16 b worn approximately at chest levelat the bottom of the rib cage, provides greatest sensitivity for readingthe signal from RC-WVM implant 12. As one possible example, core wire 82may be made up of 300 strands of twisted 46 AWG copper wire with a totallength in the range of approximately 0.5-3 m. For an antenna beltconfigured to stretch to accommodate patient girths in the range ofabout 700 to 1200 mm, the total length of core wire 82 may beapproximately 2 m. In some embodiments, it may be preferable to placethe antenna belt more caudally, with the height approximately at theheight of the patient's elbows when standing.

Many ways of providing a workable buckle for such an antenna belt may bederived by persons of ordinary skill based on the teachings containedherein. Factors to be considered in designing such a buckle includephysical security, ease of manipulation by persons with reduceddexterity and protection from electrical shock by inadvertent contactwith the electrical connectors. As an example, buckle 74 is comprised oftwo buckle halves, inner half 74 a and outer half 74 b as shown in FIG.3D. Buckle 74 provides not only physical connection for the belt ends,but also electrical connection for the antenna circuit formed by corewire 82. With respect to the physical connection, buckle 74 isrelatively large in size to facilitate manipulation by persons withreduced dexterity. A magnetic latch may be employed to assist closure,for example magnetic pads 86 a on inner buckle half 74 a connect tomagnetic pads 86 b correspondingly disposed on buckle outer half 74 b.If desired, the system can be configured to monitor for completion ofthe belt circuit and therefore detect belt closure. Upon confirmation ofbelt closure, the system may be configured to evaluate the signalstrength received from the implant and an assessment made if thereceived signal is sufficient for a reading to be completed. If thesignal is insufficient, an instruction may be provided to reposition thebelt to a more optimal location on the patient.

Electrical connection of core wire 82 may be provided by recessedconnector pins disposed on opposed connector halves 88 a and 88 b.Connection of power and data link 24 may be provided, for example,through a coaxial RF cable with coaxial connectors (e.g., SMA plugs) onbuckle 74 and control system 14. As just one possible example, aconvenient length for the power and data link, using a conventional, 50Ohm coax cable, is about 3 m.

As mentioned above, use of a single coil antenna as in antenna belt 16 brequires switching the antenna between transmit and receive modes. Suchswitching is executed within control system 14, an example of which isschematically depicted as control system 14 a in FIG. 4. In thisembodiment, control system 14 a includes as functional modules 20 asignal generator module 20 a and a receiver-amplifier module 20 b. Thesefunctional modules, along with transmit/receive (T/R) switch 92 providefor the required switching of antenna belt 16 b between the transmit andreceive modes.

FIG. 3E schematically illustrates the interaction of the magnetic field

, created by antenna belt 16 b, with RC-WVM implant 12. Both antennabelt 16 b and implant 12 are generally disposed around an axis (A). Forbest results with a belt-type antenna, the axes around which each aredisposed will lie in a substantially parallel orientation and, to theextent practicable, will lie coincident as shown in FIG. 3E. Whenproperly oriented with respect to one another, current (I) in core wire82 of antenna belt 16 b generates magnetic field

, which excites the coil of implant 12 to cause it to resonate at itsresonant frequency corresponding to its size/geometry at the time ofexcitation. An orientation between the antenna belt 16 b and implant 12as shown in FIG. 3E minimizes the power necessary to excite the implantcoil and produce a readable resonant frequency response signal.

As with any RF coil antenna system, the antenna and system must bematched and tuned for optimum performance. Values for inductance,capacitance and resistance and their interrelationship should becarefully considered. For example, the coil inductance determines thetuning capacitance while the coil resistance (including the tuningcapacitance) determines the matching capacitance and inductance. Giventhe relatively low power of the disclosed systems, special attention isgiven to these aspects to ensure that an adequately readable signal isgenerated by RC-WVM implant 12 upon actuation by the driving magneticfield. With an adjustable girth belt such as antenna belt 16 b (or withdifferent size antenna belts), additional considerations are presentedbecause of the variable or different lengths of antenna coil controlledby the control system. To address these considerations, separatetuning-matching circuits 94, 96 (FIG. 4), as are understood in the art,are provided in signal generator module 20 a and receiver-amplifiermodule 20 b, respectively.

Using conventional coax cable for RF-power transmission, as is describedabove in one embodiment of power and data link 24, optimal RF powertransfer between the antenna and the control system is achieved when thesystem and antenna impedances are matched to 50 Ohm real resistance.However, in the embodiment described above, resistance of antenna belt16 b is generally far below 50 Ohm. Transformation circuits, as part oftuning-matching circuits 94, 96 can be used to transform the antennaresistance to 50 Ohm. In the case of antenna belt 16 b it has been founda parallel capacitor transformation circuit is efficient for thispurpose.

In one example of tuning using the system components heretoforedescribed, a series capacitor was used, which, in conjunction with amatching capacitor, forms the total resonance. Using measured values asset forth below in Table II, a target resonance frequency was computedat 2.6 MHz based on the inductance and capacitance. Considering theinductance variation with stretching of antenna belt 16 b at 2.6 MHz,the resonance frequency was measured to vary only from about 2.5 MHz toabout 2.6 MHz for change in length between 1200 mm and 700 mmcircumferences of antenna belt 16 b, respectively. Considering theresistance of 11.1 Ohm, the Q-factor of the cable/belt assembly computesto be 3. Such a low Q-factor translates to a full width of the pulse athalf maximum of 600 kHz. This is far less than the variation of theresonance frequency due to stretching of the belt from 700 mm to 1200 mmcircumference. Tuning values for antenna belt 16 b were thus determinedat 2.6 MHz with C_(match)-2.2 nF and C_(tune)-2.2 nF.

TABLE II Example of measured values for antenna belt 16b Belt stretchedto 28 cm dia. around water bottle Point of Resistance Inductancemeasurement [Ohm] [10⁻⁶H] Measured at buckle 0.3 1.69 terminals with nocable connected Measured at output 11.1 3.03 of T/R switch 92 with 3mcoax cable connected

While it could be expected that a variable length antenna, such asincluded in antenna belt 16 b might present difficulties in tuning andmaintaining the antenna tuning as the length changed, it was discoveredthat with the present configuration this was not the case. As describedabove, by intentionally employing a cable for power and data link 24that has a relatively large inductance compared to the antennainductance, the proportional change in the inductance due to changes inbelt diameter are small enough not to degrade performance.

Referring again to FIG. 4, in addition to tuning-matching circuit 94,signal generator module 20 a includes components that produce the signalneeded for excitation of RC-WVM implant 12. These components includedirect digital synthesizer (DDS) 98, anti-aliasing filter 100,preamplifier 102 and output amplifier 104. In one embodiment, the signalgenerator module 20 a is configured to produce an RF burst excitationsignal with a single, non-varying frequency tailored to a specificRC-WVM implant that is paired with the system (exemplary waveformsillustrated in FIGS. 5A and 5B). The RF burst comprises a predefinednumber of pulses of a sinusoidal waveform at the selected frequency witha set interval between bursts. The RF burst frequency value selectedcorresponds to the natural frequency of the paired RC-WVM implant 12that would produce a lowest amplitude in the implant reader output. Bydoing this, optimum excitation is achieved for the worst case of implantresponse signal.

In an alternative implementation, control system 14 excites antennamodule 16 at a pre-determined frequency that is within an expectedbandwidth of the paired RC-WVM implant 12. The system then detects theresponse from the paired RC-WVM implant and determines the implantnatural frequency. Control system 14 then adjusts the excitationfrequency to match the natural frequency of the paired implant andcontinues to excite at this frequency for a complete reading cycle. Aswill be appreciated by persons of ordinary skill, frequencydetermination and adjustment as described for this embodiment may beimplemented via software using digital signal processing and analysis.

In another alternative implementation, each individual RF burstcomprises a continuous frequency sweep over a predefined range offrequencies equal to the potential bandwidth of the implant (FIG. 6A).This creates a broadband pulse that can energize the implant at allpossible natural frequencies (FIG. 6B). The excitation signal cancontinue in this “within burst frequency sweep mode” or the controlsystem can determine the natural frequency of the sensor and adjust totransmit solely at the natural frequency.

In a further alternative implementation, the excitation comprises atransitory frequency sweep over a set of discrete frequency valuescovering the potential bandwidth of the paired RC-WVM implant 12. Thefrequency is sequentially incremented for each RF burst and the RMSvalue of the RC-WVM implant response is evaluated after each increment.Control system 14 then establishes the frequency that produces themaximum amplitude in RC-WVM implant response and continues exciting thepaired RC-WVM implant at that frequency until a drop of a predefinedmagnitude is detected and the frequency sweep is re-started.

In yet another implementation, the excitation signal is composed of apre-defined set of frequencies, wherein each remain constant. Controlsystem 14 excites antenna module 16 (and hence the paired implant) byapplying equal amplitude at all frequency components. The system detectsthe response from the paired implant and determines its naturalfrequency. Control system 14 then adjusts the relative amplitude of theexcitation frequency set to maximize the amplitude of the excitationfrequency that is closest to the natural frequency of the pairedimplant. The amplitude of the other frequencies are optimized tomaximize the response of the paired implant while meeting therequirements of electro-magnetic emissions and transmission bandwidthlimitations.

In another implementation, direct digital synthesizer (DDS) 98, may beprovided as a multi-channel DDS system to generate a simultaneouspre-defined number of discrete frequencies belonging to the estimatedoperational bandwidth of the paired RC-WVM implant 12 as shown in FIGS.7A and 7B. The magnitude of each frequency component thus may beindependently controlled to provide the optimum excitation to a specificRC-WVM implant 12 based on its individual coil characteristics.Additionally, the relative amplitude of each frequency component can beindependently controlled to provide optimum excitation to the implant,i.e., the amplitude of the frequency component is selected in such a waythat in the worst case for the paired implant to transmit a responsesignal (i.e., most compressed) the excitation signal is maximized. Inthis arrangement all outputs from the multi-channel DDS system 98 aresummed together using summing amplifier based on a high speedoperational amplifier.

In yet another implementation, signal generator module 20 a can beconfigured to provide pulse shaping as illustrated in FIG. 8. Arbitrarywaveform generation based on direct digital synthesis 98 is employed tocreate a pulse of a predefined shape, the spectrum of which is optimizedin order to maximize the response of the paired RC-WVM implant 12. Themagnitude of the frequency components that result in decreased ring backsignal amplitude is maximized while the magnitude of the frequencycomponents that result in increased ring back signal amplitude isreduced, in order to obtain an approximately constant output signalamplitude and thus improved response from RC-WVM implant 12.

Referring again to FIG. 4, receiver-module 20 b, in addition totuning-matching circuit 96, includes components, e.g., single end inputto differential output circuit (SE to DIFF) 106, variable gain amplifier(VGA) 108, filter amplifier 110 and output filters 112, for implantresponse detection, data conversion and acquisition for signal analysis.During the receive period, the T/R switch 92 connects the antenna belt16 b to the receiver-amplifier 20 b, via the tuning and matching network96. The response signal induced by the implant 12 in the antenna belt 16b is applied to a unity-gain single ended to differential amplifier 106.Converting from single-ended to differential mode contributes toeliminate common mode noise from the implant response signal. Since theamplitude of the implant response signal is in the microvolts range, thesignal is fed, following conversion from single-ended to differential,into a variable gain differential amplifier 108 that is able to provideup to 80 dB (10000 times) voltage gain. The amplified signal is thenapplied to a active band-pass filter-amplifier 110 to eliminateout-of-band frequency components and provide an additional level ofamplification. The resulting signal is applied to passive, high-orderlow pass filters 112 for further elimination of out-of-band highfrequency components. The output of the filter is fed into the dataconversion and communications module 22. The data conversion andcommunications module 22 includes components to provide data acquisitionand transfer from the electronic system to the external processing unit.A high-speed analog-to-digital converter (ADC) 114 converts the outputsignal of the receiver module 20 b into a digital signal of a predefinednumber of bits (e.g., 12 bits). This digital signal is transferred inparallel mode to microcontroller 116. In one implementation, a levelshifter circuit is used to match the logic levels of the ADC to themicrocontroller. The data outputted by the ADC is sequentially stored ininternal flash memory of the microcontroller. To maximize the datathroughput, direct memory access (DMA) is used in this process.Microcontroller 116 is synced with the direct digital synthesizer 98, sodata acquisition starts when an RF burst is transmitted for excitationof implant 12. Once triggered, the microcontroller captures a predefinednumber of samples (e.g. 1024). The number of samples multiplied by thesampling period defines the observation window over which the responsesignal from implant 12 is assessed. This observation window is matchedto the length of the response signal from implant 12, which depends onthe time constant of the signal decay.

As a means of noise reduction, the response signal of the implant 12 isobserved a predefined number of times (e.g., 256), and the averageresponse is then computed. This approach greatly contributes toincreasing the signal-to-noise ratio of the detected signal.

The average response is then transmitted to an external interface device18 (e.g., laptop computer) by means of communications module 118.Different approaches can be taken for this. In one embodiment, thecommunication is performed using the UART interface from themicrocontroller and external hardware is employed to convert from UARTto USB. In a second embodiment, a microcontroller with USB drivingcapabilities is employed, and in this case connection with the externalinterface device is achieved by simply using a USB cable. In yet anotherimplementation, the communication between the microcontroller and theexternal interface device is wireless (e.g. via Bluetooth).

The system is to be powered by a low voltage power supply unit (PSU),consisting of a AC-DC converter with insulation between mains input andoutput providing a minimum of 2 Means of Patient Protection (MOPP) asper Clause 8 of IEC 60601-1:2005+AMD1:2012. In this way, the powersupply provides protection from electrocution to the user. The PSU isable to accommodate a wide range of mains voltages (e.g., from 90 to 264VAC) and mains frequencies (e.g., 47 to 63 Hz) to allow operation of thesystem in different countries with different mains specifications.

Control system 14 a as described above utilizes a software-basedfrequency detection. Thus, in terms of signal transmission, once theexcitation frequency is optimized, system 10 employing control system 14a with signal generator module 20 a operates in open loop mode, i.e.,frequency or frequencies and amplitude of the transmit signal are notaffected by RC-WVM implant 12 response. On the receive side, usingamplifier-receiver module 20 b, control system 14 a detects the responsesignal from RC-WVM implant 12 and such signal is digitized using ahigh-speed data converter. The raw digitized data is subsequentlytransferred to a processing unit (e.g., laptop computer or otherequipment microcontroller) and digital signal analysis techniques (e.g.Fast Fourier Transform) are applied to establish the frequency contentof the signal. Thus, one advantage of using these software-basedtechniques is that phased-lock loop (PLL) circuits or similar circuitsare not used or required in control system 14 a.

A further component of the overall RC-WVM system as described herein isthe RC-WVM implant delivery system. FIGS. 9A-D schematically illustrateaspects of intravascular delivery systems for placing RC-WVM implants 12at a desired monitoring location within the IVC, which may generallycomprise delivery catheter 122 including outer sheath 124 and pusher 126configured to be received in the lumen of outer sheath 124. In general,insertion of devices into the circulatory system of a human or otheranimal is well known in the art and so is not described in detailherein. Those of ordinary skill in the art will understand after readingthis disclosure in its entirety that RC-WVM implants 12 can be deliveredto a desired location in the circulatory system using, e.g., a loadingtool to load a sterile RC-WVM implant into a sterile delivery system,which may be used to deliver an RC-WVM implant to the IVC via a femoralvein or other peripheral vascular access point, although other methodsmay be used. Typically RC-WVM implant 12 will be implanted using adelivery catheter, delivery catheter 122 being an illustrative examplethereof, and the RC-WVM implant will be optimized for delivery throughas small a catheter as possible. To facilitate this, bends at theimplant crown sections 40 (elsewhere referred to as ears or collectively“sensor construct end portions”) may be small-radius bends to facilitatea low profile when packed into the delivery catheter as shown. In onealternative, pusher 126 may be provided with a stepped distal end 128having a reduced diameter end portion 130 configured to engage the innerperimeter of RC-WVM implant 12 when compressed for delivery. For implantembodiments employing anchors such as anchors 48 in FIG. 2 or anchors 48s in FIG. 34A et seq, end portion 130 may be configured to engage aninner perimeter defined by the anchors in the compressed configurationas illustrated in FIG. 9B. Alternatively, pusher distal end 128 may beprovided with a straight, flat end or other end shape configured tocooperate with a specific RC-WVM implant and anchor design. For example,as shown in FIG. 9D, RC-WVM implant 12 t with anchor frame 650 (see,e.g., FIGS. 40 and 41) may be deployed with a flat distal end pusher128, which bears against crown sections 40 of implant 12 t, with anchorframe 150 disposed opposite pusher 128.

In one deployment option, an RC-WVM implant may be inserted from aperipheral vein such as the femoral or iliac vein into the IVC to bepositioned at a monitoring location between the hepatic and renal veins.It will be understood that the implant also may be introduced from othervenous locations. Depending on implant configuration, when placed in theIVC for fluid status monitoring, specific orientation of RC-WVM implant12 may be required to optimize communication with the belt readerantenna coil. To facilitate desired placement or positioning, the lengthand diameter of RC-WVM implant 12 may be designed so that it graduallyexpands (“flowers”) as it is held in position with the pusher 126 andthe sheath 124 is withdrawn, as schematically illustrated in FIG. 9C.Here, RC-WVM implant 12 is shown partially deployed with the distalcrowns already engaging the IVC wall while the proximal crowns are stillcontained within sheath 124. Such a gradual, partial deployment helpsensure that RC-WVM implant 12 is properly positioned in the IVC. Thesensor length to vessel diameter ratio (where the length is alwaysgreater than the vessel diameter) is also an important design factor toensure that the sensor deploys in the correct orientation in the IVC. Ina further alternative, distal end 128 of pusher 126 may be configured toreleasably retain the anchors or a proximally oriented portion of theimplant before it is fully deployed from outer sheath 124 so that it maybe retracted for repositioning as needed. For example, small, radiallyextending studs may be provided near the end of end portion 130, whichengage behind the proximal crowns of implant 12 so long as it iscompressed within outer sheath 124 whereby the implant may be pulledback in from a partially deployed position, but self-releases from thestuds by expansion when fully deployed after positioning is confirmed.Conventional radiopaque markers may be provided at or near the distalends of outer sheath 124 and/or pusher 126, as well as on RC-WVM implant12 to facilitate visualization during positioning and deployment of theimplant. Typically, where anchor features are employed, the implant willbe positioned with the anchor features proximally oriented so theanchors are the last portion deployed in order to facilitate correctorientation within the IVC and potentially allow for pull back andrepositioning as may be needed. Once the implant is fully deployed,delivery catheter 122 may be withdrawn from the patient, leaving implant12 as a discrete, self-contained unit in the vessel without attachedwires, leads, or other structures extending away from the monitoringlocation.

Example 1

Systems as described herein have been evaluated in pre-clinical testingusing RC-WVM implant 12 a (as in FIG. 2), an antenna belt similar toantenna belt 16 b (as in FIG. 3) and control system 14 a (as in FIG. 4).The implants were deployed into ovine IVCs using delivery systems 122(as in FIG. 9B) using standard interventional techniques. Deployment wasconfirmed angiographically, using intravascular ultrasound and using theantenna belt.

FIGS. 10A, 10B and 10C illustrate, respectively, the raw ring downsignal, detection of the maximum frequency and conversion of this to anIVC area using a reference characterization curve. FIG. 10A shows theraw ring down signal in the time domain with the resonant response ofthe RC-WVM implant decaying over time. Modulation of the implantgeometry results in a change in the resonant frequency which can be seenas the difference between the two different plotted traces. FIG. 10Bshows the RC-WVM implant signal as converted into the frequency domainand plotted over time. The maximum frequency from FIG. 10A is determined(e.g., using fast Fourier transform) and plotted over time. The larger,slower modulation of the signal (i.e., the three broad peaks) indicatethe respiration-induced motion of the IVC wall, while the faster,smaller modulation overlaid on this signal indicate motion of the IVCwall in response to the cardiac cycle. FIG. 10C shows the frequencymodulation plotted in FIG. 10A converted to an IVC area versus timeplot. (Conversion in this case was based on a characterization curve,which is determined through bench testing on a range of sample diameterlumens following standard lab/testing procedures.) FIG. 10C thus showsvariations in IVC area at the monitoring location in response to therespiration and cardiac cycles.

The ability of RC-WVM implant 12 (in this case, implant 12 a) to detectIVC area changes as a result of fluid loading is demonstrated in FIGS.10D and 10E. In one example, the results of which are shown in FIG. 10D,after placement of RC-WVM implant 12 in the ovine IVC and confirmationof receipt of the implant signal, a fluid bolus of 100 ml at 10 ml/s wasadded to the animal. The grey band in FIG. 10D indicates theadministration of the fluid bolus. As reflected by the decreasingfrequency ring-back signal from RC-WVM implant 12, the added fluidvolume caused the IVC to expand, and with it the implant, which in turncauses a change in the inductance of the implant thus changing thefrequency of its ring-back response to excitation. In another example,with results shown in FIG. 10E, the operating table was tilted to shiftfluid within the animal. Starting from the left in FIG. 10E, the firstgrey band indicates the time when the table was initially tilted.Tilting of the table caused fluid to shift away from the IVC, causingthe IVC to reduce in diameter, and thus increasing the frequency of thering-back signal of RC-WVM implant 12 as it moved to a smaller diameterwith the IVC. The second grey band indicates the time when the table wasreturned from tilted to flat. At this point, fluid shifts back into theIVC, causing it to increase in size with the added fluid volume and thusreduce the frequency of the ring-back signal as explained above.

These output signals thus demonstrate the detection of modulation of theIVC with respiration. In particular, it will be appreciated thatembodiments of the present invention can thus provide an unexpectedlypowerful diagnostic tool, not only capable of identifying gross trendsin IVC geometry variations, but also capable of discriminating inreal-time between changes in IVC geometry arising from respiration andcardiac function.

Alternative Patient Care Systems Based on RC-WVM Implants DisclosedHerein

FIG. 11A schematically illustrates an alternative system 10 a configuredto provide patient care based on fluid status monitoring using an RC-WVMimplant 12 positioned at a monitoring location in the IVC as elsewheredescribed herein. Using RC-WVM implant 12, measurements of IVC diameteror area by implant 12 may be made continuously over one or morerespiratory cycles to determine the variation in patient fluid volumeover this cycle. Further, these measurement periods may be takencontinuously, at preselected periods and/or in response to a remotelyprovided prompt from a health care provider/patient.

Antenna module 16 may be configured to communicate via wireless or wiredconnection 24 with control system 14, as elsewhere described herein.Data and information collected by control system 14 may be communicatedultimately to a healthcare provider device 131 via hard wired links suchas telephone or local area networks 132 or through Internet orcloud-based systems 133. Personal communication devices 134, such assmart phones or tablets, also may be used for communication with, or asalternatives to, other communications devices and modes describedherein. Healthcare provider device 131 may be configured with anappropriate user interface, processing and communications modules fordata input and handling, communications and processing, as well astreatment and control modules, which may include treatment algorithms asdescribed herein for determining treatment protocols based on collectedIVC diameter or area measurements, and systems for automated remotecontrol of treatment devices based on determined treatment protocols aselsewhere described herein. Examples of such treatment devices include,but are not limited to, dialysis machine 135 and drug delivery devices136. Examples of treatments include, when measured dimensions fallwithin the hypovolemic warning zone, administration of fluids orvaso-constricting drugs, and when measured dimensions fall within thehypervolemic warning zone, dialysis or administration of diuretics orvasodilating drugs.

IVC physical dimension data and/or fluid volume state informationderived therefrom may also be communicated directly to the patientthemselves, along with therapy advice based on this data and usingpre-determined algorithms/implanted medical devices. Communicationsprotocols throughout the system may include bidirectional communicationsto permit a healthcare provider (or other appropriately trained operatorat another point in the system) to alter overall monitoring protocolsexecuted at the monitoring device or, for example, to request additionalqueries by the monitoring device outside the current operationalprotocol.

Other embodiments include systems for patient self-directed therapy, forexample with IVC volume metrics data utilized directly by the patientwith or without clinician overview, e.g., for self-administration ofdrugs or other therapies. Such systems may also be implemented for homedialysis and/or peritoneal dialysis. Wireless communication between theIVC monitor and the patient's or healthcare provider's cell phone orcomputer would allow continuous or periodic transmission of IVC data andthe use of software applications to provide alarms or reminders,graphically present trends, suggest patient actions, drug dosageoptions, or treatment system settings, and allow communication withphysicians.

FIG. 11B schematically illustrates another exemplary system, which may,in one alternative, incorporate patient self-directed therapy. As shownin FIG. 11B, system 10 b provides for communication between the patienthome system 137, cloud storage 133, a patient management system 138, aphysician alert system 139, and optionally a hospital network 140. Datatransmission from the patient home system 137 to the cloud 133 forstorage and access facilitates remote access for clinical and nursingteams. In patient self-directed therapy embodiments, patient's home mayinclude home therapy devices 141, which may independently access cloudstorage 133, and based on predetermined limits/treatment algorithms,indicate patient self-administration of medications or drug delivery 136or home dialysis machines 135. In such a system a patient with wirelessimplant 12 may receive prompts from a cell phone or other device in thehome at specific time intervals or may utilize data (D) generated byother patient monitoring devices such as blood pressure, heart rate orrespiration monitors that also communicate with the home device asinputs to decision-making algorithms, and may transmit data to cloud 133for storage. System 10 b may also include communication links (direct,networked or cloud-based) with such other monitoring devices to receivedata (D) inputs used in setting warning zones and alert limits andassessing patient fluid state. Further inputs may be made by a userthrough a user interface, which may be, for example, configured as partof patient management system 138. User inputs may include additionalpatient-specific information such as patient age, sex, height, weight,activity level, or health history indicators.

In response to a prompt from system 10 b to take a reading, the patientwould position him/herself with respect to or on antenna module 16 asappropriate to communicate with selected RC-WVM 12. A user interface ofcontrol system 14, or, in one possible alternative, personalcommunication device 134 may provide sequential prompts and/orinstructions to the patient.

Varying levels of response may be generated by home system 137 dependingon IVC measurements received from RC WVM implant 12 and as may beinterpreted in light of other patient data (D). Minimal responses may beprovided if the patient fluid status is within acceptable ranges and noaction is required. Mid-level responses may include warnings or tocontact healthcare providers or prompts for medication administration orchanges in home drug delivery, or home dialysis. Consistentlyout-of-range or increasing readings would prompt response escalation toclinical intervention. Patient treatment protocols, in general, may bebased on the applicable standards of care for disease state managementas informed by diagnostic information reported by RC-WVM implant 12 andsystem 10. Specific examples of treatment protocols designed to takeadvantage of the unique capabilities of RC-WVM implant 12 are providedin Applicant's co-pending international application no.PCT/US2017/046204, filed Aug. 10, 2017, entitled “Systems And MethodsFor Patient Fluid Management”, which is incorporated by referenceherein. When home dialysis or drug delivery is prompted, it may becontrolled directly in a closed-loop system as described above or may becontrolled by the patient with prompts from the system. Patient data (D)and IVC measurements from RC-WVM implant 12 also may be communicatedcontinuously or periodically by system 10 b to cloud storage 133 andfurther communicated to a remote patient management system 138.Functionality for system 10 b may be largely contained in home system137 or in patient management system 138 or appropriately distributedacross the network. Optionally, patient-related data including sensorresults and patient health and fluid states also may be communicated toor accessible by a hospital network 140. System 10 b also may receivepatient-related data, including for example, medical records related topast therapies and medical history.

When a patient condition is recognized by system 10 b as outsideacceptable limits, an alert may be generated by physician alert system139. Information supporting the alert condition may be communicated, forexample, through patient management system 138 to physician alert system139. Physician alert system 139 may reside at a healthcare provideroffice and/or may include a mobile link accessible by the healthcareprovider remotely to permit communication 142 between the healthcareprovider and the patient. Communication 142 between healthcare providerand patient may be network, Internet or telephone-based and may includeemail, SMS (text) messaging or telephone/voice communication. Physicianalert system 139 allows the healthcare provider to review logs of IVCmeasurements and medication changes over time and make decisionsregarding therapy titration, and in critical cases, hospital admissions,remote from the patient.

Exemplary system embodiments 10 a and 10 b are each illustrated,respectively, in FIGS. 11A and 11B with various system functionsassigned to particular functional elements of the systems. For the sakeof clarity of the disclosure, not all possible distributions orcombinations of functions in functional elements across the system aredescribed. As will be appreciated by persons of ordinary skill, otherthan the function of the RC-WVM implant itself, all functions may bedistributed among functional elements in any number of arrangements asbest suited to a home or clinical application and the intended locationof sensor reading function, e.g., in a home or hospital setting. Forexample, all system functions (except implant-specific functions asmentioned) may be contained in a single functional unit in the form of astand-alone patient management system. Alternatively, functions may behighly distributed among mobile devices networked with secure cloudcomputing solutions. For example, control system 14 may communicatedirectly with a patient-owned smart phone to receive signals indicatingIVC physical dimension measurements and, in turn, transmit those signalsvia WiFi or cell network to the cloud for distribution to further mobiledevices in the possession of healthcare providers. Hand-held devices134, such as tablets or smart phones, may communicate directly withcontrolled-treatment delivery devices, or such devices may be controlledby a self-contained patient management system. Further, processingnecessary for operation of the system also may be distributed orcentralized as appropriate, or may be duplicated in multiple devices toprovide safety and redundancy. Thus, the specific arrangement of thefunctional elements (blocks) in the schematic presentations of theillustrative examples in FIGS. 11A and 11B are not to be considered aslimiting with respect to possible arrangements for distribution ofdisclosed functions across a network.

As mentioned above, various care algorithms may be developed based onsystems 10 a and 10 b. For example, in one scenario, a first, home-carealgorithm governs interactions in the home system including periodic IVCdiameter/area measurements using RC-WVM implant 12 and dictates whetherto maintain current therapies or to change therapies within the scope ofhome-care team capabilities. As long as IVC volume metrics stay withinpredefined limits, the first, home-care algorithm continues to governmonitoring and treatment. However, if monitored parameters, for exampleIVC volume metrics, exceed the predefined limits, then an alert isgenerated that engages a second, healthcare-provider algorithm. Such analert may be generated internally by home system 137, or may begenerated in patient management system 138 (or physician alert system139) based on monitored data communicated by home system 137 andreceived by the other systems either periodically or on a continuousbasis. In one embodiment, an alert is received initially by aphysician's assistant or heart failure nurse who can triage thesituation through patient management system 138 locally or remotely. Atthis initial level the assistant or nurse may elect to generate amessage for communication 142 to the patient through the network relatedto modulation of therapy or other parameters such as level of physicalactivity. However, if triage indicates the alert to represent a morecritical event, the physician may be alerted through physician alertsystem 139. Multiple layers of care and review based on measured IVCvolume metrics are thus provided to efficiently manage patient fluidstatus and where possible avoid hospitalizations.

RC-WVM Implant Design Considerations and Alternative Implant Embodiments

It will be appreciated that the measurement of dimensional changes inthe IVC presents unique considerations and requirements arising from theunique anatomy of the IVC. For example, the IVC is a relatively lowpressure, thin-walled vessel, which changes not simply its diameter, butits overall shape (cross-sectional profile) in correspondence to bloodvolume and pressure changes. Rather than dilating and constrictingsymmetrically around its circumference, the IVC expands and collapsesprimarily in the anterior-posterior direction, going from a relativelycircular cross-section at higher volumes to a flattened oval-shapedcross-section at lower volumes. Thus embodiments of RC-WVM implants 12must monitor this asymmetrical, low-pressure collapse and expansion inthe A-P direction without excessive radial constraint, yet must alsoengage the vessel walls with sufficient force to anchor the implantsecurely and prevent migration. Accordingly, RC-WVM implant 12 must becapable of collapsing with the vessel in the A-P direction from agenerally circular cross-section to an oval or flattened cross-sectionwithout excessive distortion of the vessel's natural shape. Theserequirements are achieved according to various embodiments describedherein by appropriate selection of material compliance and configurationsuch that the coil measurement section of RC-WVM implant 12 ismaintained in contact against the IVC wall without undue radial pressurethat may cause distortion thereof. For example, RC-WVM implants 12according to embodiments described herein may exert a radial force inthe range of about 0.05N-0.3N at 50% compression. In anotheralternative, potentially increased security of positioning may beachieved without compromising measurement response by physicallyseparating anchoring and measurement sections so as to move possibledistortions of the vessel wall due to anchoring a sufficient distancespaced from the measurement section so as not to affect measurements.

RC-WVM implants 12 as described may be configured in various structuressuch as collapsible loops or tubes of formed wire with resilientsinusoidal or “Z-shaped” bends, or as more complex collapsible shapeswith more resilient regions such as “spines” joined by relatively lessresilient regions such as “ears.” Each structure is configured based onsize, shape and materials to maintain its position and orientationthrough biasing between resilient elements of the implant to ensurecontact with the vessel walls. Additionally or alternatively, anchors,surface textures, barbs, scales, pin-like spikes or other securementmeans may be placed on the structure to more securely engage the vesselwall. Coatings or coverings also may be used to encourage tissuein-growth. In some embodiments it may be preferable to configurespecific portions of the structure, for example the coil spines, as theposition-maintaining engagement portion in order to reduce any effect ofthe biasing force on movement of the vessel walls as sensed at the coilears, or vice-versa. In yet other embodiments, separate anchoringstructures may be coupled to a coil-measurement portion of the implant.Such anchoring structures may comprise hooks, expandable tubularelements, or other tissue-engaging elements which engage the vesselupstream or downstream of the coil portion so as to minimize anyinterference with the natural expansion or contraction of the vessel inthe area of the coil itself. Sensing modalities and positioning isdescribed in more detail below.

When RC-WVM implant 12 is energized it must generate a signal ofsufficient strength to be received wirelessly by an external system. Inthe case of a variable induction circuit, the coil which transmits thesignal to the external receiver must maintain a tubular shape or centralantenna orifice of sufficient size, even when the vessel is collapsed,such that its inductance is sufficient to generate a field strong enoughto be detected by an external antenna. Thus, in some embodiments, it maybe desirable that the variable inductor have a collapsing portion whichdeforms with the expansion and collapse of the vessel, and anon-collapsing portion which deforms relatively little as the vesselcollapses and expands. In this way, a substantial portion of the coilremains open even when the vessel is collapsed. In other embodiments,the coil may be configured to deform in a first plane containing theanterior-posterior axis while deflecting relatively little in a secondorthogonal plane containing the medial-lateral axis. In still otherembodiments, a first inductive coil may be provided to expand andcollapse with the vessel, and a separate transmit coil, which deformssubstantially less, provided to transmit the signal to the externalreceiver. In some cases the transmit coil also may be used as ananchoring portion of the implant.

Turning to specific alternative RC-WVM implant embodiments disclosedherein, a first exemplary alternative embodiment is RC-WVM implant 12 c,shown in FIG. 12A. Implant 12 c may comprise a “dog-bone-like” shape asshown with coil portion 142 and capacitor portion 144. Implant 12 c maycomprise an electrically conductive wire or bundle of wires that iswound or otherwise formed into a single continuous coil comprisingmultiple turns or loops having an oval or rounded rectangular shape. Itmay be advantageous to use “Litz” wire, which has multiple independentlyinsulated strands of wire, for the coil, since that may enhance theinductance of the implant. The coil is configured to be oriented suchthat the longer dimension of the generally rectangular loops extendlongitudinally in a cranial-caudal direction within the IVC. The wire orgroup of wires may be wound multiple times in a continuous overlappingmanner such that the rectangular loops each are defined by two or moreparallel strands or bundles of wire about their periphery. Therectangular loops have central regions bounded by two or morelongitudinal wires 146 forming spines 148 approximately defining acentral plane running longitudinally in a cranial-caudal direction. Thiscentral region is configured to be disposed in a plane generallyperpendicular to the anterior-posterior axis of the vessel, and remainsrelatively un-deformed as the vessel collapses and expands in theanterior-posterior direction. The longitudinal elements may engageopposing walls of the vessel. At the caudal and cranial ends of thecentral regions of the rounded rectangles, the wire or wires form twolobes or a pair of coil ears 150 that flare outwardly away from eachother and from the central plane of the implant in the anterior andposterior directions, as shown in FIG. 12A. Coil ears 150 are configuredto engage opposing anterior and posterior walls of the vessel and toleave the central lumen of the vessel completely unobstructed for flowof blood as indicated by the arrows.

As the IVC changes shape, the longitudinal wires may move closertogether or farther apart, and coil ears 150 may also move closertogether or farther apart, thereby changing the inductance of the coil.The ears may be separated by about 1 cm to about 5 cm at the apex of thecurved ends of the ears. RC-WVM implant 12 c, as adapted for an averageIVC size, may be about 2.5 cm to 10 cm long. It may be appreciated thatas the IVC collapses in the anterior-posterior direction, coil ears 150deform inwardly thereby changing the inductance of the coil. However,the central region of the coil remains relatively un-deformed andmaintains sufficient size that the inductance of the coil is high enoughto produce a field sufficiently strong for external detection. Capacitorportion 144 in this embodiment includes discrete capacitor 152 tocomplete the L-C circuit. Capacitor portion 144 may be alternativelylocated in a number of locations, such as distal to coil ears 150, oralong one of spines 148.

As described above, the IVC in a typical monitoring region between thehepatic and renal veins is relatively compliant, and tends to collapseinto a non-circular oval-shaped cross-section, which is wider in themedial-lateral direction than it is in the anterior-posterior direction.A feature of “dog-bone” style implant such as RC-WVM implant 12 c isthat spines 148 create more stiffness in the plane of the central regionof the coil which causes the device to rotationally auto-orient aroundthe longitudinal axis of the vessel with the two spines along the medialand lateral walls, and coil ears 150 flaring anteriorly and posteriorly.Typically, a RC-WVM implant 12 thusly configured will assume an unbiasedimplanted configuration in which the distance between the spinespreferably corresponds to the natural medial-lateral dimension of theIVC at current blood volume such that the implant does not distort thevessel from its natural shape. In one alternative, overall the diameterof RC-WVM implant 12 may be somewhat oversized as compared to the vesseldiameter at its secured location so it is always relatively biasedoutward against the vessel walls. In such a case, when the IVCcollapses, the A-P dimension reduces and the M-L dimension increases,although the M-L increase is generally much less than the A-P collapse,the oversizing maintains vessel wall contact and secure positioning. Aselsewhere discussed, resiliency of the coil/wires forming the implantmust be selected in this case also so as to move with the vessel withoutdistorting measurements based on vessel wall movement.

A further alternative embodiment of RC-WVM implant 12 is the “x-bow”shaped implant 12 d, shown in FIG. 12B. Like “dog-bone”-shaped RC-WVMimplant 12 c, “x-bow”-shaped RC-WVM implant 12 d may comprise anelectrically conductive wire or group of wires of types previouslydescribed formed into coil portion 154 and capacitor portion 156.However, rather than being formed into a rounded rectangular shape as inRC-WVM implant 12 c, “x-bow”-shaped RC-WVM implant 12 d may be wound orotherwise formed into two ellipsoid shapes disposed in intersectingplanes to form two sets of coil ears 158 as shown. In oneimplementation, an “x-bow”-shaped RC-WVM implant 12 d may be formed bywinding on a mandrel or otherwise forming an ellipsoid shape with one ormore wires in a single plane and then bending one or more turns of theone or more wires out of that plane into an ellipsoid shape in anotherplane to form an overall shape like that illustrated in FIG. 12B. Acapacitor element such as discrete capacitor 160 may be convenientlyplaced in capacitor portion 156 at one of the intersections of the “X”or at one of the ends of ears 158. An implant configured as RC-WVMimplant 12 d might preferably be placed in the IVC with coil ears 158oriented as described above (against the anterior-posterior walls of theIVC). Blood flow through the open central lumen of the implant wouldfollow the direction of the large arrows in FIG. 12B.

Similar to “dog-bone”-shaped RC-WVM implant 12 c, “x-bow”-shaped RC-WVMimplant 12 d deforms with the vessel walls in the anterior-posteriordirection while having relatively little deformation in the mediallateral direction. RC-WVM implant 12 d is thus able to deform with theIVC as it collapses but retains an open coil shape in the medial-lateraldirection to maintain a high level of inductance, thus being capable ofproducing a field of sufficient strength to be detected by an externalreceiver.

In other embodiments, a tether or stent-like structure may be used toanchor RC-WVM implant 12 in a predetermined location while allowing itto very gently press against the walls of the vessel desired to bemonitored. An important issue that must be taken into consideration isthe fact that implants in veins or arteries can modify the flexibilityor resiliency of the vein or artery to the point that changes in theshape of the veins or arteries that may be expected to be measurableusing such implants may not take place or may be severely attenuated dueto the shape of, function of, or vascular response to the implant.Accordingly, it is important that the implant have sufficient stiffnessto anchor itself in the vessel while simultaneously allowing naturalexpansion and contraction of the vessel walls at the location(s) wherethe implant is measuring vessel dimension. In the implants describedabove, for example, the wall-engaging ears of the coils must havesufficient compliance/flexibility and resilience to move in and out withthe vessel walls without excessive distortion or attenuation of thenatural wall motion.

As shown in FIG. 12C, RC-WVM implant 12 e is an example of analternative implant embodiment employing a stent-like structure foradditional stability or anchoring security. RC-WVM implant 12 e isformed as an “x-bow” type implant similar to RC-WVM implant 12 d,discussed above, but with added sinusoidal, expandable and collapsiblewire support 162 around the center of the implant and secured at theopposed coil wire crossing points 164. Wire support is insulated fromthe coil wires forming coil ears 158 so as not to interfere with theelectrical performance of the implant. As one example, wire support 162may be formed of a nitinol wire or laser cut shape as used for the frameof the implant itself (see, e.g. frame 44 in FIGS. 2 and 2B or frames244 or 246 in FIG. 20A or 20B, respectively). The stent-like structureof wire support 162 allows it to expand and collapse with the implantand assists in uniform expansion and localization of anchoring forceaway from coil ears 158.

In another RC-WVM implant 12 alternative embodiment, an “x-bow”-shapedRC-WVM implant similar to RC-WVM implant 12 d shown in FIG. 12B may beformed with two separate coils in orthogonal planes to allow measurementof the vessel dimension in two axes, i.e. in both the anterior-posteriordirection and the medial-lateral direction. FIGS. 13A, 13B and 13Cillustrate such an alternative embodiment. As shown therein, RC-WVMimplant 12 f is formed with two separate coils 166, 168 to form twoseparate, independent resonant circuits tuned to two differentfrequencies. RC-WVM implant 12 f thus includes two capacitors 170, 172,one for each circuit. With two separately tuned coils, RC-WVM implant 12f has the ability to discriminate between changes in dimension along twoperpendicular axes, one through coil ears 174, indicated by arrows E inFIG. 13A, and the other through coil spines 176, indicated by arrows Sin FIG. 13C. The two separate resonant circuits can be separatelyenergized so as to resonate independently. The two measurements may needto be taken using two input waveforms having different frequencies sothat the outputs subsequently generated by RC-WVM implant 12 f can bedifferentiated by the external receive antenna. Alternatively, coils ofdifferent geometry, or capacitors of different capacitance, could beused to produce different resonant frequencies for a given inputwaveform. An antenna module 16 with planar antenna coils, for example asshown in FIG. 22A or 22B may be preferred with such a two coil typeimplant such as RC-WVM implant 12 f. With the implant shaped as shown inFIGS. 13A-C, coupling is anterior-posterior. Use of two separately tunedcoils also provides an opportunity to exploit the mutual inductance ofthe coils. With two coils together as disclosed, the inductance of eachcoil may stay constant or equal compared to one another. Mutualinductance equals the first inductance multiplied by the secondinductance and a coupling factor (M=L1*L2*k).

FIG. 13D shows signal response of a prototype RC-WVM implant 12 f. Theprototype was constructed with two 0.010″ Nitinol frames, each insulatedwith PET heatshrink material. The overall frame size was approximately25-30 mm diameter and approximately 60 mm long. A first coil on oneframe comprised three turns of 60 strand 46 AWG copper Litz wire, with asoldered connection to a 15 ηF capacitor. A second coil opposite framecomprised four turns of 60 strand 46 AWG copper Litz wire, with asoldered connection to a 5.6 ηF capacitor. PET heatshrink insulation wasprovided around each coil and the two coils joined together in the x-bowconfiguration shown in FIGS. 13A-C with epoxy. The three plots in FIG.13D represent (from left to right) the signal response for theuncompressed implant, the signal response for compression along thespines (arrows S) where the two frequency peaks increase in unison, andthe signal response for compression at the coil ears (arrows E) wherethe gap between the frequency peaks increases. The independent responsefrom each of the two coils is clearly represented by the two distinctfrequency peaks in each plot and therefore the A-P and M-L distensionsof the IVC can be understood.

FIGS. 14A and 14B illustrate another alternative RC-WVM implant 12 g,also with two separate coils that may be tuned to different frequencies.In this embodiment, coils 178 and 180 are mounted onresilient/compressible frame members 182 and 184. Coils 178 and 180 maybe formed on frames with multiple turns of fine Litz wire as with otherRC-WVM implant embodiments described herein and are generallyrectangular in shape with slightly upturned ends 186 and 188. Coils 178and 180 run perpendicular to loops in frame members 182 and 184. Framemembers 182 and 184 also have electrical breaks as described above withrespect to, e.g., frame 44. RC-WVM implant 12 g as shown does notinclude discrete capacitors and hence relies on the inherent capacitanceof the implant coils to complete the L-C circuit. However, discretecapacitors could be added in each coil as an alternative.

Other embodiments of RC-WVM implant 12 may be adapted to balance theanchoring and measuring requirements by providing separate,longitudinally spaced measurement and anchor sections. Such embodimentssplit the anchoring and measurement into two discrete regionslongitudinally separated from each other a sufficient distance that theanchoring section does not distort or constrain the vessel in the regionbeing measured. The radial force characteristics of the measurement andanchoring sections will determine the spacing required, in certainembodiments, where the radial force of both sections is relatively low,the spacing can be reduced to as little as 5 mm. Examples of RC-WVMimplant embodiments with separate measurement and anchor sections areshown in FIGS. 15A-B, 16A-B and 17A-B. One such alternative embodimentis RC-WVM implant 12 h, shown in FIG. 15A. As shown therein, anchorsection 190 (also an antenna section as explained below) can be stiffer,of different geometry, with its expanded shape set to a larger diameterthan measurement section 192 to securely anchor RC-WVM implant 12 h.Anchor section 190 may be comprised of nitinol or other suitablematerial to increase resilience and/or stiffness while still allowingcollapse for deployment. In some embodiments, a separate antenna coilmay be integrated with or coupled to the anchor section, as describedbelow, to enable separation of vessel measurement from signaltransmission/reception.

As mentioned, embodiments of RC-WVM implant 12 with separate anchor andmeasurement sections also may employ the anchor section as an antennacoil. RC-WVM implant 12 h, shown in FIG. 15A, is an example of such anembodiment. Anchor section 190 and measurement section 192 are providedas two mechanically separate, but electrically continuous coils, one forvessel measurement and a second as an antenna for signal receptionand/or transmission. Advantageously, separation of the measurement coil194 from antenna coil 196 allows the antenna coil to be less affected bychanges in vessel size and to have a shape and size selected to maximizethe transmitted signal (i.e. magnetic field) generated by it. Moreover,antenna coil 196 may be configured to anchor the implant in the vessel,or may be integrated or coupled to an anchoring element, withoutaffecting the performance of measurement coil 194. Antenna coil 196 maythus have more turns of more strands of Litz wire and a differentgeometry and size than measurement coil 194 to optimize both anchoringand communication with the external antenna. In the RC-WVM implant 12 hexample, anchor section 190 is formed as multiple loops in a generallyoval shape, shaped to engage the inner walls of the vessel. Measurementsection 192 is formed, for example, as a sinusoidal “z” shape, which maycomprise a thinner, lower radial force nitinol frame, with fewer turnsof higher gauge (thinner) wire, or fewer strands of Litz wire thanantenna coil 196. Measurement section 192, forming measurement coil 196,is highly compliant and minimizes distortion of the vessel's naturalexpansion and collapse so as to accurately perform the measurementfunction. Measurement coil 194 may have a variety of other geometries,such as sinusoidal, square wave, or other open-cell designs, but ingeneral will not have closed-cells or other electrical connectionsbetween the successive loops of the coil, which could create problematiceddy currents. RC-WVM implant 12 h is also provided with discretecapacitor 198 on strut section 200 joining the anchor/antenna sectionand measurement section/coil.

A further alternative embodiment for RC-WVM implant 12 involves the useof two capacitors to “double tune” the device. One example of such anembodiment is RC-WVM implant 12 i, shown in FIG. 15B. In thisembodiment, first capacitor (CT) 202 is associated with measurement coil(Ls) 204, while second capacitor (CA) 206 is associated with antennacoil (L_(A)) 208, allowing independent tuning of the measurement andantenna circuits to optimize dynamic range, field strength and signalduration. These capacitors can be selected such that the deflection ofmeasurement coil 204, which is a low percentage of the overallinductance of RC-WVM implant 12 i and would normally result in only asmall shift of the resonant frequency, can be made to have a largerdynamic range and therefore produce a more detectable shift in thisfrequency. At the same time, the resonant frequency of antenna coil 208can be optimized for reception by the external antenna. With such anarrangement antenna coil 208 also may be configured as an anchor sectionas discussed above.

FIGS. 16A-B illustrate further alternative RC-WVM implants 12 j and 12k. RC-WVM implant 12 j, in FIG. 16A, includes sinusoid element sensor210 composed as previously described with respect to other similarlyshaped sensor coils. Sensor element 210 is attached via elongateisolation connector 212 to anchor section 213. Sensor element 210 alsocommunicates with antenna module 16. Anchor section 213 is provided witha curved wire anchor element 214 configured to engage with the IVC walland fix the implant at a monitoring location. Isolation connector 212isolates sensor element 210 from any distortions or irregularities thatthe IVC wall may be subjected to by anchor section 213. AlternativeRC-WVM implant 12 k, shown in FIG. 16B, employs two separate sinusoidelements 216, 217, formed in one continuous coil using techniques asdescribed herein. Sinusoid element 216 exerts a lower radial force inresistance to diameter changes and is thus designed to operate as theRC-WVM sensor coil. Sinusoid element 217 is configured to exert a higherradial force and thus forms an anchor section and also may be configuredfor communication with antenna module 16. Anchor isolation means 218 maybe formed as a wire connection portion between elements 216 and 217.

FIGS. 17A-B illustrate a further alternative RC-WVM implant 12 m,wherein FIG. 17A shows an oblique view and FIG. 17B shows a normal view.Coil sensor element 220 is provided as elsewhere described herein; inthis case having a somewhat wider cross-section as a result of coilwires formed around a rectangular cross-section laser cut frame. Anchorsection 222 is displaced from sensor element 220 by anchor isolationmeans 223. Both anchor section 222 and anchor isolation means 223 may beformed, for example, from nitinol wire. Locating anchor section 222separately from sensor element 220 allows for the use of higher radialforce in the anchor section without impacting the sensed region of theIVC. Anchor section 222 may rely on radial force alone for fixation ormay incorporate individual, pointed anchors. Anchor section 222 may beconfigured as in many embodiments, including any other anchor/anchorsection disclosed herein. As shown in FIGS. 17A-B, anchor section 222employs “ears” 224 that are self-biasing outward to widen and engagewith the vessel wall.

FIG. 18 illustrates a further alternative RC-WVM implant 12 n. In thisembodiment, two sinusoidal, “Z”-shaped coils 226, 228 are joined atconnections 230 by two pairs of elongate members 232. Coils 226, 228 maybe formed on different thicknesses frames of nitinol wire thus resultingin different radial forces, i.e., a lower force end for measurement anda higher force end for anchoring. Elongate members 232 thus also serveas anchor isolation means between sensor and anchor coils. The sensorcoil may be a two turn coil, constructed from multi-strand Litz wire (aselsewhere described herein) and the anchor coil may also have a largearea to further provide strong communication with antenna module 16.

FIGS. 19A and 19B illustrate a further alternative RC-WVM implant 12 p.In this embodiment, two turn coil 234, which may be formed from wrappedLitz wire as elsewhere described, is separated from dual sinusoidalnitinol anchoring structure 236, 237. Outwardly curved “ears” 238 ofcoil 234 are configured to engage the IVC wall with less force to formthe sensor or measurement element, and relatively, the large area ofcoil 234 optimizes communication with antenna module 16. Dual nitinolanchoring structures 236, 237, provide a separated, higher radial force,anchoring portion. Thus, a flat portion 240 of coil 234 provides ananchor isolating function.

In any embodiment of RC-WVM implant 12 described herein, it may beadvantageous to form the coil portion of the implant with multi-strandedwire or cable comprising a plurality of separately insulated strandswound or braided together to optimize the performance with highfrequency alternating current. In some embodiments, the electricallyconductive wire or wires used in the implant may comprise Litz wire inwhich the separately insulated strands of wire are braided or woundtogether in a particular prescribed pattern to optimize AC currenttransmission by optimizing for the high frequency “skin effect”. Theindividual wire insulation could be PTFE, polyester, polyurethane,nylon, or polyimide, among others. An additional insulated jacket may beprovided around the entire multi-stranded wire or cable in order toprovide electrical insulation from blood, which could otherwise renderthe implant suboptimal or unreliable under some circumstances, and tobind the Litz wire to the frame. Such additional insulation may beprovided in the form of PET (polyethylene terephthalate), ETFE, FEP,PE/PP, TPE, polyurethane, silicone, polyimide, or other material, andmay be provided on the wires of an RC-WVM implant and/or to encaseRC-WVM implant 12 in its entirety. Due to the use of high frequencyelectromagnetic signals, more, or different, insulation may need to beprovided for the electrical portions of RC-WVM implant 12 than may berequired for other types of implants or electrical devices.

In some embodiments, nitinol frame such as frames 244 and 246, shown inFIGS. 20A and 20B, respectively, may be used to provide structuralsupport and enhanced anchoring, and to facilitate the crimping orcompression and deployment or expansion of RC-WVM implant 12 into/fromthe delivery sheath. For example, the nitinol frame may be formed in thedesired shape of the coil (using formed wire 244 or a laser cut tube orthin plate 246), and the conductive wire may then be wound coextensivelywith the nitinol frame to form the coil. Alternatively, nitinol wire andLitz wire may be co-wound or braided and then the composite cable usedto form the coil, so that the electrical inductance of the nitinol wireis added to that of the Litz wire. The structure may then be insulatedwith, e.g., silicone tubing or moulding. In other embodiments, a nitinoltube with Litz wire disposed coaxially within it (or vice versa) couldbe used; such a tube may have, for example, about a 0.020″ to 0.050″inner diameter with walls having a thickness of, for example, about0.005″ to 0.020″. In other embodiments, the coil may be formed withgold-coated nitinol wire and/or a drawn-filled tube. Any exposedsurfaces of any non-insulated portions of RC-WVM implant 12 arepreferably made from or plated with biocompatible polymers or metalssuch as gold, platinum, palladium/molybdenum or plated in thesematerials to prevent undesirable effects or health issues. Nitinol wireframe 244 includes strut sections 38 and crown sections 40 as previouslydescribed. As a wire formed frame, frame 244 has a natural break 245that occurs where the wire ends are brought together. Where needed, toavoid creating an electrical loop through the frame, the break can bebonded together with an insulating material such as epoxy to completethe frame structure.

Laser cut frame 246, as shown in FIG. 20B, is cut from a nitinol tubewhich is expanded and shape set to size including integral anchorelements 250, formed by laser cutting orifices 254 and shape setting theanchor elements 250. Frame 246 is electro-polished after cutting, beforecoil wires are wrapped as described below. When formed by cutting from atube, frame 246 will be a continuous member and thus must be cut atlocation 38 during a pre-coil wrapping stage to avoid forming anelectrical loop within the frame which could negatively impact theperformance of the coil. The cut section may then be re-joined bybonding with an insulating material such as epoxy or over-moulding witha polymer. Anchors 250 may be located on extending posts 252 withopenings 254 from which anchor elements 250 are formed. Such anchorelements may extend bi-directionally as shown or only in a singledirection. While relatively short compared to other frame dimensions,anchor elements 250 should be long enough to protrude past wire andinsulation when added to frame 246 to engage with the vessel wall forfixation. Typically, when anchor elements 250 are formed only on one endof the fame, they will be on the proximal end of the frame so as todeploy last when deployed from the delivery catheter as explained above.However, alternatively, anchor elements 250 may be formed on both endsof the frame. As shown in FIG. 20B, anchor attachment elements 250 areprovided on each proximal crown section 40 joining strut sections 38 offrame 246. Alternatively, extending posts 252 o4 other anchor attachmentpoints may be provided on fewer than all crown sections, for example onevery other crown section.

FIGS. 21A and 21B illustrate aspects of one example of a method formaking an RC-WVM implant using a wire frame such as wire frame 244 shownin FIG. 20A. After formation of the frame, it is expanded on a fixture,such as by hooks 256, to approximately a maximum diameter. The selectedwire, such as Litz wire 42, is then wrapped around the frame. Multipleparallel wraps may be made, which may have turns between crown sections40 to distribute the wire evenly and cover the frame. The wrappingobjective is to achieve an evenly distributed wire, covering the strutand crown sections 38, 40 with a consistent but thin wire coating. Inone alternative technique, the first and last wraps may be radial tobind wire 42 to the frame. After wrapping is complete, the structure isinsulated by a dip, spray or heatshrink process. Typical insulationmaterials may include silicone, TPU, TPE or PET. The method stepsheretofore described contemplate use of individually insulated Litz wirestrands. If uninsulated wire strands are to be used, then an additionalpre-wrapping step of insulating the frame itself before applying thewire may be desired. FIG. 21B illustrates the wrapped frame 244 after itis removed from fixture hooks 256. Another technique involves laying themultiple strands of thin wire next to each other in a continuous loopwith as many turns as called for in the design. Such loops may bewrapped around the frame only a small number of times compared to themethod above, e.g. as few as one or two times. The entire assembly maythen be held together with a suitable external insulation as described.

The number of turns of wire used to form a coil portion of RC-WVMimplant 12 embodiments may be optimized to provide enough conductivematerial to allow the use of lower capacitance value capacitors in orderto enable the use of a physically smaller capacitor, thereby minimizingimplant size. The preferred number of turns will depend on variousfactors including the diameter of the coil, the size and number ofstrands of wire or cable, the strength of the field produced by thetransmit antenna, the sensitivity of the receive antenna, the Q value ofthe capacitor, and other factors. Such coils could have anywhere from 1to 10 or more turns (each turn being a complete 360 degree loop of thewire around the frame), and preferably have at least 2 such turns. Forexample, Litz wire used in an RC-WVM implant 12 embodiment may have 180strands of 46 AWG (0.04 mm wire), but could include anywhere from 1 to1000 strands, and the strands could be about 0.01 to 0.4 mm in diameter.

Alternative System Embodiments, Components and Modules

Alternative embodiments 16 c and 16 d for antenna module 16 areillustrated, respectively, in FIGS. 22A and 22B. As shown, in FIG. 22A,control system 14 generates input waveforms and receives signals backfrom RC-WVM implant 12 as elsewhere described herein. In particular,signal generator module within control system 14 drives figure-eighttransmit coil 258, which energizes RC-WVM implant 12. Due to the LCcircuit formed by the wires of RC-WVM implant 12, the implant will thenresonate and produce magnetic fields of its own as a consequence of theinduced current. The magnetic fields produced by RC-WVM implant 12 canthen be measured using receive coil 260, which is monitored viaamplifier-receiver module within control system 14, which may thendeliver data to remote system 18. In alternative antenna embodiment 16c, receive coil 260 comprises a single, square coil lying in the samegeneral plane as the transmit coils so as to be properly oriented togenerate a current when a magnetic field is generated by the implant.Under the well-known right-hand rule, when a current flows through thetransmit coils, a magnetic field will be generated in a directionperpendicular to the plane of each coil. By causing the current to flowin opposite directions around each transmit coil, the magnetic fieldforms a toroidal shape flowing from one transmit coil into the patient'sbody, through the inductive coil of the implant, and back out of thepatient through the other transmit coil. This arrangement produces ageometric decoupling of the transmit and receive coils, as is describedin greater detail below in connection with FIG. 25B. Also, as discussedelsewhere in more detail, it will be noted that the implant should beoriented such that the field produced by the transmit coils passesthrough the center of the implant's inductive coil. This generates acurrent flowing through the inductive coil which, due to the capacitorin the circuit, resonates at a specific frequency based upon the sizeand shape of the coil. This current in turn generates a field whichpasses out of the implant perpendicular to the plane of the inductivecoil, and through the external receive coil, generating a currenttherein. The frequency of this current can be measured and correlatedwith vessel diameter. In alternative antenna embodiment 16 d, transmitcoil 262 also comprises two square coils, but in this case receive coil264 comprises two round coils, one each disposed within a transmit coil.Again, the transmit and receive coils are disposed in the same plane asdescribed above.

Example 2

Systems as described herein have been evaluated in pre-clinical testingusing RC-WVM implant 12 c as shown in FIG. 12A, and antenna module 16 das schematically depicted in FIG. 22B. The implants were deployed intoporcine IVCs using femoral access and standard interventional technique.Deployment was confirmed angiographically and using intravascularultrasound. External antenna module 16 d was placed under the animal andring-back signal obtained.

FIG. 23A illustrates the raw ring-back signal obtained in pre-clinicaltesting at multiple time points, and FIG. 23B illustrates how thissignal can be converted from frequency to time domain using Fouriertransform. The coil resonance modulation can then be converted to vesseldimension through calibration. In FIG. 23B, the frequency modulatesbetween approximately 1.25 to 1.31 MHz. It was then possible tocorrelate this frequency shift to an IVC dimensional change bycharacterizing the compression of the coil under specific displacements(and their associated resonant frequencies) as described below. The stepnature of the frequency signal may be improved by increasing the Q ofthe signal, providing longer ring-down and facilitating betterresolution of the signal. The strength of the signal will also beoptimized with iterations of Litz wire and insulation.

The raw voltage signal in FIG. 23A is as received from the RC-WVMimplant, which was positioned in an anterior-posterior orientation ofthe spines. An antenna module as depicted schematically in FIG. 22B,employing a figure-eight circular shape coil was used as transmit coiland a figure-eight square coil as receive coil “TX” and “RX”,respectively. These were coupled and an Arduino controller (or any othermicrocontroller could be used) was used to switch the receive coil onand off resonance to improve transmit and receive decoupling. Thedecompressed resonance frequency of the implant coil was 1.24 MHz at 25mm diameter. Fully compressed, the resonance frequency of the implantcoil was 1.44 MHz. FIG. 23B shows the resonance frequency as determinedfor each measurement as a function of time with a clear variation offrequencies in the expected compression range between 1.24 and 1.44MHz-1.25 MHz being nearly fully decompressed (24 mm diameter=only 1 mmof compression) and 1.31 MHz being about 50% compressed (16.25 mmdiameter=8.75 mm of compression). Based on these results, modulation ofresonant frequency of the RC-WVM correlated with IVC diameter variationwas observed.

Further alternative examples of configurations and components forcontrol system 14 and antenna module 16 are shown in FIGS. 24A through26C. FIGS. 24A and 24B illustrate examples of excitation and feedbackmonitoring (“EFM”) circuits that can be used to excite the L-C circuitin a RC-WVM implant and monitor the response of the RC-WVM implant tothat excitation. These circuits may be used as components in alternativecontrol systems 14. After the receive coil in an EFM circuit receivessignals corresponding to the response of the RC-WVM implant to theexcitation previously generated using the EFM circuit, those signals maybe processed digitally to convert the signal to the frequency domainusing a Fast Fourier Transform (“FFT”) algorithm, a zero-crossingalgorithm, or other methods. After such processing is complete, thefrequency having the highest magnitude within the calibration frequencyrange of the implant (i.e. all possible frequencies that the implant cancontain such as for instance 1.4 to 1.6 Mhz) is determined and shouldcorrespond to the resonant frequency of the LC circuit in the RC-WVMimplant. By continually monitoring the frequency having the highestmagnitude in signals received from the LC circuit of the RC-WVM implantin response to discrete excitations of a transmit coil connected to theEFM circuit, the EFM circuit can be calibrated to translate a frequencyshift in signals received from the L-C circuit of the RC-WVM implantinto a dimension, area and/or collapsibility index of the vein or arteryin which the RC-WVM implant is disposed. In some implementations, aheartbeat and/or other physiological signals (e.g. respiration, cardiacheart beat) can be derived from small variations in frequency ormagnitude or shape of signals received from the RC-WVM implant afterbeing excited by a transmit coil attached to an EFM circuit. In someembodiments, magnitude variations in the signals received from theRC-WVM implant can be used to validate frequency variations in thesignals received from the RC-WVM implant through cross-correlation orother methods of correlating signals. FIG. 25A illustrates one exampleof a tuning and detuning network, which may be used in antenna module 16in conjunction with excitation and feedback monitoring (“EFM”) circuitsas exemplified by FIGS. 24A and 24B, discussed. In an antenna module 16with this configuration, TX coil transmits the excitation signal toRC-WVM implant 12 and RX coils receives the ring-back signal from theimplant.

In some embodiments, where a single antenna-coil may be used for boththe transmit and receive signals, antenna module 16 includes a switchingmechanism to alternate between transmission and reception, therebyeliminating interference between the transmitted signal and the receivedsignal. Examples of such switches are the passive and active diodeswitches shown in FIGS. 28A and 28B. In other embodiments, in whichantenna module 16 employs separate transmit and receive coils, thereceive coil may be geometrically decoupled from the transmit coil toeliminate interference between the two, even when operatingsimultaneously. In one such embodiment, shown in FIG. 25B, receive coil278 forms a single square shape surrounding all or a portion of bothtransmit coils 280 resulting in a geometric decoupling of the coils. (Asimilar arrangement is also depicted schematically in FIG. 22A.) Use ofa smaller antenna for transmit reduces emissions, while use of a largerreceiver coil maximizes signal-to-noise ratio. Such an arrangementexploits the optimum geometry for transmitting from a planar,figure-eight loop into an orthogonally oriented RC-WVM implant while thereceive function can be used to maximize the magnetic flux caught fromthe implant in the receive coil. This arrangement can be helpful whereloop-to-loop coupling is not possible, e.g., when a belt antenna is notused. The coils are tuned to resonance frequency and matched to sourceimpedance (e.g., 50 Ohm).

Advantageously, this allows simultaneous transmission and reception offields to/from the implant to maximize signal strength and duration, andpotentially eliminate complex switching for alternating betweentransmission and reception. Notably, in some implementations, single orplural circular or other-shaped transmit and/or receive coils may beused, the transmit and receive coils may be disposed in the same planeor different planes, and the area enclosed by the transmit coil may belarger or smaller than the area enclosed by the receive coil. Thetransmit and receive coils may be formed using copper tape or wire orcould be implemented as a portion of a printed circuit board.

The transmit and receive coils used for exciting RC-WVM implant 12 andreceiving the implant ring-back signal in response to that excitation,respectively, should be tuned (matched and centered) on the particularRC-WVM implant's L-C circuit resonant frequency range. In exemplaryembodiments, a signal generator may be used to generate a sine waveburst of 3 to 10 cycles at 20 Vpp with a frequency selected to maximizethe response of the RC-WVM implant L-C circuit. The signal generator maytransmit a burst at whatever rate provides a clinically adequatemeasurement of the variation in the vessel dimensions; this could beevery millisecond, every ten milliseconds, or every tenth of a second.It will be understood that a variety of waveforms may be used includingpulse, sinusoidal, square, double sine wave, and others so long as thewaveform contains the spectral component corresponding to the resonantfrequency of the implant. Geometric decoupling, damping, detuning,and/or switching may be used to prevent the transmit pulse signals frombeing picked up by the receive coil while the transmit coil istransmitting.

FIG. 26A schematically depicts an alternative signal generation module20 a as excitation waveform generator 282, which generates the RFenergizing signal transmitted to RC-WVM implant 12 (not shown) byantenna module 16 (not shown). In this embodiment, Direct DigitalSynthesis (DDS) waveform synthesizer 284 (with clock signal from clock285) provides a low voltage RF burst signal the parameters of which areconfigurable by external input through microcontroller 286 usingfrequency adjustment control 288. Microcontroller 286 also includes syncconnection 289 to receiver-amplifier module 20 b. LCD controller 290communicates with microcontroller 286 to cause LCD display 292 todisplay the selected frequency. Microcontroller 286 thus initializes andprograms the DDS 284 allowing configuration of output waveformparameters (e.g., frequency, number of cycles per RF burst, intervalbetween burst, frequency sweep, etc.). Output from DDS 284 (lowamplitude RF signal) is applied to high order, anti-aliasing low passfilter 294. The filtered signal from filter 294 is applied to anamplification chain, which may comprise preamplifier 296 and outputamplifier 298 in order to present a flat frequency response over thefrequency band of interest.

FIG. 26B schematically depicts an alternative receiver-amplifier module20 b as receiver chain 300, which conditions the ring-back signalreceived from RC-WVM implant 12 (not shown) by antenna module 16 (notshown) after excitation by signal generation module 20 a. In thisexample, a single-ended low-noise preamplifier (not shown) provides flatresponse over the frequency band of interest and input to low noiseamplifier 302 is matched to the receiver antenna of antenna module 16(not shown). Unity gain amplifier 304 provides single-ended todifferential conversion of the signal into a programmable gain,differential to differential stage in order to provide a high level ofamplification. Variable gain amplifier 306 is controlled by theDigital-to-analog (DAC) output 308 of microcontroller 310, which issynced to signal generation module 20 a, for example excitation waveform generator 282 shown in FIG. 26A, at sync connection 312 so that thegain is minimized during the excitation period to minimize coupling ofexcitation signal in the receiver circuitry. A low-pass or band-passdifferential filter/amplifier 314 of an order of at least four (4)provides rejection of noise and unwanted signals. Output differentialamplifier 316, the gain of which is selectable so that the magnitude ofthe output signal covers as much dynamic range as possible of the dataconversion stage communicates with hardware-based frequency detection318 to assert the frequency of the response signal provided by thesensor. Frequency detection 318 provides an output to ananalog-to-digital converter (not shown).

FIG. 26C schematically depicts an alternative communication module 22 asdata converter 320, which processes the signal from receiver-amplifiermodule 20 b to allow for interpretation of the measurement signals fromRC-WVM implant 12 (not shown). In this example, data conversion isachieved by means of high-speed, high-resolution, parallel outputAnalog-to-Digital converter (ADC) 322. Coupling from receiver-amplifiermodule 20 a to ADC 322 is performed by coupling transformer 324 tominimize noise. ADC 322 may be specified to provide LVCMOS or CMOScompatible output to easily interface with a wide range of commerciallyavailable microcontrollers. In one embodiment, low voltage CMOS (LVCMOS)to CMOS level shifter 326 is employed for interfacing purposes withmicrocontroller 328. ADC 322 provides a conversion complete signal tosync with the data capture stage.

FIGS. 27A and 27B show further alternative embodiments for antennamodule 16 as alternative belt antennas 16 c and 16 d, respectively. Inorder to accommodate patients of different girth, belt antenna 16 cincludes fixed portion 330 and one or more extension portions 332 ofvarying lengths. Fixed portion 330 includes male and female connectors334, 336, which may connect directly to form a smallest size belt byboth mechanically securing the belt and electrically completing theantenna coil. Extension portions also include male and female connectors334, 336 so they may be connected into a fixed belt portion thusproviding different sizes and completing mechanical and electricalconnections. In order to tune the antenna and match it to the RC-WVMimplant and signal generation circuitry (e.g. modules 20 a), one optionis to provide fixed portion 330 and each different length extensionportion 332 with a fixed inductance, resistance and capacitance suchthat total parameters for the completed belt antenna 16 c are knowncorresponding to each set length. Signal generation module 20 a ofcontrol system 14 (not shown) can thus be adjusted as needed for aparticular length belt and patient girth to provide necessary tuning andmatching. Instead of different length extension portions, belt antenna16 d uses multiple connection points 340 for closure portion 342. Eachconnection point 340 corresponds to a different length belt toaccommodate a range of patient girths. At one end, main portion 344 andclosure portion 342 include clasp 346 with male and female connectors toprovide mechanical closure and electrical circuit completion. Closureportion 342 includes connector 348 opposite clasp 346, which isconnectable to each connection point 340 to change the belt length. Eachconnection point 340 also includes fixed compensation inductor circuit350 matched and tuned to the corresponding belt length to provideautomatic tuning and matching without the need to compensate withcontrol system 14.

FIGS. 28A and 28B illustrate diode switches suitable as transmit/receive(T/R) switch 92 of control system 14 for use when an antenna module 16is employed with a single coil antenna as discussed above. Passive diodeswitch 352 in FIG. 28A comprises crossed diodes 354, 356. The diodes areautomatically switched open by larger voltages applied during transmitand closed when smaller voltages are read during receive. In oneexample, the switch threshold is set at about 0.7V such that the switchis open at voltages above the threshold and closed at voltages below it.Active diode switch 360 in FIG. 28B comprises PIN diode 362, directcurrent (DC) blocking capacitors 364, RF blocking choke coils 366, andDC power supply 368. Diode 362 is switched open and closed by externallycontrolled logic (not shown). The DC voltage change is confined to thePIN diode 362 and an RF choke path created by blocking capacitors 364.As a result, the RF signal cannot penetrate the DC current path due tothe RF chokes and the signal to antenna module is thus turned off duringa receive mode.

FIGS. 29A and 29B illustrate further alternative belt antennaembodiments of antenna module 16. FIG. 29A shows an embodiment in whichantenna module 16 does not employ a wired connection for power and commlink 24, but instead wirelessly connects alternative belt antenna 16 eto control system 14. In this embodiment power and comm link 24 andantenna belt 16 e utilize a second pair of coupling coils 370, 372 totransmit the signals between the belt and the power and comm link. Apartfrom its second coupling coils 372 for communication with matched coil370 on power and comm link 24, antenna belt 16 e may be configured asdescribed for any previous antenna belt embodiment. FIG. 29B describes afurther alternative embodiment in which control system 14 is powered bybattery and incorporated into belt antenna 16 f to provide an overallsystem that is less restrictive for the patient. In this embodiment,control system 14 contains a wireless module which is used tocommunicate the required information to base station 374, which in turncommunicates with a remote system (i.e. cloud data storage/wirednetwork) as previously described. The belt-mounted battery in thisembodiment may be charged via non-contact near field communication,wireless charging by being placed on charging pad 376, which in turnwould receive its power directly from base station 374 or from AC powersource 378. Also in this embodiment other aspects of antenna belt 16 fmay be configured as described above for other antenna belt embodiments.

RC-WVM Embodiments with On-Board Power and Electronics and RelatedControl Systems

In some situations it may be desirable to remove the necessity forexternal transmit and receive antennas, increase the communicationsdistance of the RC-WVM implant and/or communicate with another implantedmonitor/device. FIGS. 30A and 30B are block diagrams illustrating twoalternative on-board electronics systems. FIGS. 31A and 31B depictalternative wireless implants 12 q and 12 r, including electronicsmodules, which may contain on-board electronics systems, for example, asshown in FIGS. 30A and 30B.

In one alternative, as exemplified by FIG. 30A, on-board electronicssystem 380 include primary battery 382 to increase communicationdistance. Other modules of electronics system 380 may include powermanagement module 384, driver circuit 386 to drive the wireless implantcoil at pre-programmed intervals and frequencies, and currentamplifier/buffer 388 to interface with the wireless implant coil. Inthis case, battery 382 provides energy used to excite the implant coiland cause it to resonate at its resonant frequency (or to produce ameasurable inductance change as explained below), but with higher powerdue to the power supply being on board (rather than using an externaltransmit coil/antenna). A stronger signal may allow a receive coil of anantenna module to be located further away (for example, under or besidethe bed) from the primary coil of an RC-WVM implant, thus giving greaterflexibility in positioning of patient and external device. In such anembodiment, there may be no need for the external transmit coil, only anexternal receive coil of the antenna module is used. In an optionalalternative, RF power harvesting 390 may be employed to capture andharness an external RF signal, power a super capacitor and then performas above. Further features possible in such an embodiment may includebattery capacity and power budget estimation, or battery down selectfrom available implant batteries.

In another alternative, as exemplified in FIG. 30B, on-board electronicssystem 392 includes primary battery 394 to provide energy to excite orotherwise power the wireless implant coil. Excitation or power deliverymay be manually initiated or in response to a signal from optionalwake-up circuit 396. Power management module 398 communicates withmicrocontroller 400, which is interfaced with inductance measurementcircuit 402 (which may include ADC and firmware to measure inductance),and serial data port 404 to send digital data, optionally throughwireless transmitter 406 if required. In one option, microcontroller 400interfaces to an analog to digital controller (“ADC”) and inductancemeasurement circuit 402 digitizes the inductance and ports this data toa serial data port 404 for wireless transmission to a sub-cutaneous bodyimplant (e.g., implant 420 in FIG. 32). Additional features in such anembodiment may include battery capacity and power budget estimation.

Illustrative examples of wireless implants 12 q and 12 r employingon-board electronics systems are shown in FIGS. 31A and 31B. Bothimplants 12 q and 12 r include an electronics module 410 containedwithin a sealed capsule/container 412, which is secured to the resilientsensor construct to electrically communicate with the implant coil.Wireless implant 12 q is depicted as employing a sinusoidal or “zig-zag”coil 414 with a similar construction and function to the coils ofimplants 12 a and 12 b, shown in FIGS. 2 and 2A. Wireless implant 12 ris depicted as employing a “dog-bone” configured coil 416 with ears 417having a similar construction and function to implant 12 c shown in FIG.12A. Note that the arrow in FIG. 31B illustrates direction of blood flowthrough the implant. Alternatively, any other implant 12 disclosedherein may be adapted with an electronics module such as module 410.

Another advantage of on-board electronics systems, such as system 392,is that the on-board system may be used to determine the resonantfrequency and transmit a signal to a sub-cutaneous cardiacmonitor/device (such as Medtronic LINQ or Biotronik BioMonitor). Thesubcutaneous cardiac monitor/device may be preexisting in the patient ormay be implanted along with the RC-WVM implant. This architecture allowsthe device to potentially take multiple readings at pre-set time pointsor as indicated by triggers such as an accelerometer. FIG. 32schematically depicts wireless implant 12 q or 12 r wirelesslycommunicating 418 with subcutaneous cardiac monitor/device 420. In thisdepiction, the wireless implant may include within electronics module410 an on-board electronics system such as system 392 as describedabove. The on-board electronics system may be configured to communicatedirectly with the communications interface of device 420 withoutnecessitating changes in that interface.

In yet a further alternative embodiment, when utilized with an on-boardpower supply as a part of an on-board electronics system, such assystems 380 or 392) wireless implants such as implants 12 q, 12 r, orother configurations disclosed herein, may be configured as a variableinductor without the necessity to include a specifically matchedcapacitance to create a tuned resonant circuit. In this case, theon-board electronics system applies a current to the implant sensor coiland then measures changes in inductance as a result of the coil-changinggeometry in response to movement of the vascular lumen wall at themonitoring location where the implant is positioned. Signals based onthe varying inductance measurements can then be transmitted by acommunications module of the on-board electronics system, again, withoutthe necessity of specially tuned antennas. Implants employing direct,variable inductance instead of a resonant circuit with a variableresonant frequency may be mechanically constructed as elsewheredescribed herein with respect to the exemplary embodiments of RC-WVMimplants 12, except that a specific capacitance or capacitor to producea resonant circuit is not required.

Hardware and Software Examples for Computer-Implemented Components

It is to be noted that any one or more of the aspects and embodimentsdescribed herein, such as, for example, related to communications,monitoring, control or signal processing, may be convenientlyimplemented using one or more machines (e.g., one or more computingdevices that are utilized as a user computing device for an electronicdocument, one or more server devices, such as a document server, etc.)programmed according to the teachings of the present specification, aswill be apparent to those of ordinary skill. Appropriate software codingcan readily be prepared by skilled programmers based on the teachings ofthe present disclosure, as will be apparent to those of ordinary skillin the software art. Aspects and implementations discussed aboveemploying software and/or software modules may also include appropriatehardware for assisting in the implementation of the machine executableinstructions of the software and/or software module. In general, theterm “module” as used herein refers to a structure comprising a softwareor firmware implemented set of instructions for performing a statedmodule function, and, unless otherwise indicated, a non-transitorymemory or storage device containing the instruction set, which memory orstorage may be local or remote with respect to an associated processor.A module as such may also include a processor and/or other hardwaredevices as may be described necessary to execute the instruction set andperform the stated function of the module.

Such software may be a computer program product that employs amachine-readable storage medium. A machine-readable storage medium maybe any medium that is capable of storing and/or encoding a sequence ofinstructions in a non-transitory manner for execution by a machine(e.g., a computing device) and that causes the machine to perform anyone of the methodologies and/or embodiments described herein. Examplesof a machine-readable storage medium include, but are not limited to, amagnetic disk, an optical disc (e.g., CD, CD-R, DVD, DVD-R, etc.), amagneto-optical disk, a read-only memory “ROM” device, a random accessmemory “RAM” device, a magnetic card, an optical card, a solid-statememory device, an EPROM, an EEPROM, and any combinations thereof. Amachine-readable medium, as used herein, is intended to include a singlemedium as well as a collection of physically separate media, such as,for example, a collection of compact discs or one or more hard diskdrives in combination with a computer memory. As used herein, amachine-readable storage medium does not include transitory forms ofsignal transmission.

Such software may also include information (e.g., data) carried as adata signal on a data carrier, such as a carrier wave. For example,machine-executable information may be included as a data-carrying signalembodied in a data carrier in which the signal encodes a sequence ofinstruction, or portion thereof, for execution by a machine (e.g., acomputing device) and any related information (e.g., data structures anddata) that causes the machine to perform any one of the methodologiesand/or embodiments described herein.

Examples of a computing device include, but are not limited to, anelectronic book reading device, a computer workstation, a terminalcomputer, a server computer, a handheld device (e.g., a tablet computer,a smartphone, smart watch, etc.), a web appliance, a network router, anetwork switch, a network bridge, any machine capable of executing asequence of instructions that specify an action to be taken by thatmachine, and any combinations thereof.

FIG. 33 shows a diagrammatic representation of one possible embodimentof a computer-based implementation of one or more aspects of controlsystem 14 in the form of specialized computing device or system 500within which a set of instructions for causing the various modules, suchas signal generation module 20 a, receiver-amplifier module 20 b andcommunications module 22, among other systems and devices disclosedherein, to perform any one or more of the aspects and/or methodologiesof the present disclosure may be executed. It is also contemplated thatmultiple computing devices may be utilized to implement a speciallyconfigured set of instructions for causing one or more of the devices toperform any one or more of the aspects and/or methodologies of thepresent disclosure. Exemplary control system 500 includes processor 504and memory 508 that communicate with each other, and with othercomponents, via communication bus 512. Communication bus 512 comprisesall communications related hardware (e.g. wire, optical fiber, switches,etc.) and software components, including communication protocols. Forexample, communication bus 512 may include any of several types of busstructures including, but not limited to, a memory bus, a memorycontroller, a peripheral bus, a local bus, and any combinations thereof,using any of a variety of bus architectures, and may comprisecommunications module 22.

Memory 508 may include various components (e.g., machine-readable media)including, but not limited to, a random access memory component, a readonly component, and any combinations thereof. In one example, a basicinput/output system 516 (BIOS), including basic routines that help totransfer information between elements within control system 14, 500,such as during start-up, may be stored in memory 508. Memory 508 mayalso include (e.g., stored on one or more machine-readable media)instructions (e.g., software) 520 embodying any one or more of theaspects and/or methodologies of the present disclosure. In anotherexample, memory 508 may further include any number of program modulesincluding, but not limited to, an operating system, one or moreapplication programs, other program modules, program data, and anycombinations thereof.

Exemplary control system 500 may also include a storage device 524.Examples of a storage device (e.g., storage device 524) include, but arenot limited to, a hard disk drive, a magnetic disk drive, an opticaldisc drive in combination with an optical medium, a solid-state memorydevice, and any combinations thereof. Storage device 524 may beconnected to bus 512 by an appropriate interface (not shown). Exampleinterfaces include, but are not limited to, SCSI, advanced technologyattachment (ATA), serial ATA, universal serial bus (USB), IEEE 1394(FIREWIRE), and any combinations thereof. In one example, storage device524 (or one or more components thereof) may be removably interfaced withcontrol system 500 (e.g., via an external port connector (not shown)).Particularly, storage device 524 and an associated machine-readablemedium 528 may provide nonvolatile and/or volatile storage ofmachine-readable instructions, data structures, program modules, and/orother data for RC-WVM control and communication system 500. In oneexample, software 520 may reside, completely or partially, withinmachine-readable medium 528. In another example, software 520 mayreside, completely or partially, within processor 504.

Exemplary control system 500 may also optionally include an input device532. In one example, a user of control system 500 may enter commandsand/or other information into the via input device 532. Examples of aninput device 532 include, but are not limited to, frequency adjust 288(FIG. 26A), as well as other alpha-numeric input devices (e.g., akeyboard), pointing devices, audio input devices (e.g., a microphone, avoice response system, etc.), cursor control devices (e.g., a mouse), atouchpad, an optical scanner, video capture devices (e.g., a stillcamera, a video camera), a touchscreen, and any combinations thereof.Input device 532 may be interfaced to bus 512 via any of a variety ofinterfaces (not shown) including, but not limited to, a serialinterface, a parallel interface, a game port, a USB interface, aFIREWIRE interface, a direct interface to bus 512, and any combinationsthereof. Input device 532 may include a touch screen interface that maybe a part of or separate from display 536, discussed further below.Input device 532 may be utilized as a user selection device forselecting one or more graphical representations in a graphical interfaceas described above.

A user may also input commands and/or other information to exemplarycontrol system 500 via storage device 524 (e.g., a removable disk drive,a flash drive, etc.) and/or network interface device 540. A networkinterface device, such as network interface device 540, may be utilizedfor connecting control system 500 to one or more of a variety ofnetworks, such as network or cloud 28, and one or more remote devices 18connected thereto. Examples of a network interface device include, butare not limited to, a network interface card (e.g., a mobile networkinterface card, a LAN card), a modem, and any combination thereof.Examples of a network include, but are not limited to, a wide areanetwork (e.g., the Internet, an enterprise network), a local areanetwork (e.g., a network associated with an office, a building, a campusor other relatively small geographic space), a telephone network, a datanetwork associated with a telephone/voice provider (e.g., a mobilecommunications provider data and/or voice network), a direct connectionbetween two computing devices, and any combinations thereof. A network,such as network 28, may employ a wired and/or a wireless mode ofcommunication. In general, any network topology may be used. Information(e.g., data, software 520, etc.) may be communicated to and/or controlsystem 500 via network interface device 540.

Exemplary control system 500 may further include display adapter 552 forcommunicating a displayable image to a display device, such as displaydevice 536. Examples of a display device include, but are not limitedto, LCD frequency display 292 (FIG. 26A), as well as other display typessuch as a cathode ray tube (CRT), a plasma display, a light emittingdiode (LED) display, and any combinations thereof, which may display,for example, user prompts, alerts, or wave forms for excitation orresponse signals as shown in FIGS. 5A-B, 6A-B, 7A-B, 8, 10A-C and 23A-B.Display adapter 552 and display device 536 may be utilized incombination with processor 504 to provide graphical representations ofaspects of the present disclosure. In addition to a display device,control system 500 may include one or more other peripheral outputdevices including, but not limited to, an audio speaker, a printer, andany combinations thereof. Such peripheral output devices may beconnected to bus 512 via a peripheral interface 556. Examples of aperipheral interface include, but are not limited to, a serial port, aUSB connection, a FIREWIRE connection, a parallel connection, and anycombinations thereof.

Disclosure Summary

The present disclosure describes plural embodiments of implantablewireless monitoring sensors configured to sense changes in a dimensionof a body lumen within which the sensor is implanted, as well as systemsand methods employing such sensors. Aspects of disclosed sensors,systems and methods include one or more of the following, which may becombined in multiple different combinations as described herein.

For example, wireless sensor implants may be optionally configured withany of the following aspects of resilient sensor constructs, coils,variable inductance or resonance, anchor elements or electricalcharacteristics:

-   -   Resilient sensor constructs may        -   Resilient metal frame            -   Shaped wire            -   Laser cut                -   Nitinol        -   Coil            -   Plural Wire strands wrapped on frame                -   Litz wire                -   Bare wire                -    Frame insulated                -   A single wrap around frame                -   Multiple wraps around frame            -   Coil shapes                -   Rotationally symmetric shape                -    Allows placement at any rotational orientation                    without effecting responsiveness                -   Asymmetric shape to correspond to variations in                    collapse of IVC in A-P and M-L directions                -    Allows for discrimination between changes in A-P                    lumen dimension versus M-L lumen dimension                -    Different radial force in different directions to                    facilitate proper placement        -   Variable inductance            -   Resonant circuit                -   Variable inductance with fixed capacitance                -    Discrete capacitor added to circuit                -    Capacitance inherent in structure        -   Anchor elements            -   Barbs or Wires                -   Cranially oriented                -   Caudally oriented                -   Bidirectionally oriented            -   Coils as anchors            -   Anchor isolation structures to separate anchoring                aspects from sensing aspects to avoid distortion of                lumen wall at sensing location    -   Electrical characteristics of implant or resilient sensor        construct configurations        -   Capacitance selected with high Q        -   Frequency            -   Frequencies in range of 1 MHz                -   Frequency selected to Maximize Q                -   Quality factor of signal related to length of ring                    back signal            -   High frequencies                -   Permit smaller antennas                -   Require more insulation

Wireless Implant sensors or resilient sensor construct configurationsbased on one of the above frame related aspects and one of the abovecoil related aspects to provide one of a variable inductance or aresonant circuit employing variable inductance and fixed capacitance,optionally with one of the above anchor element aspects may take any ofthe following configurations:

-   -   Rotationally symmetric, sinusoidal or linked “Z-shape”        configurations as shown in FIGS. 2 and 2A.    -   “Dog bone” shaped configurations as shown in any of FIGS. 12A,        19A and 19B    -   “X-bow” shaped configurations as shown in any of FIGS. 12B and        12C    -   Separate coil configurations as shown in any of FIGS. 13A, 13B        and 13C    -   Configurations with decoupled anchoring and sensing functions as        shown in any of FIGS. 12C, 14A, 14B, 15A, 15B, 16A, 16B, 17A,        17B, 19A, 19B,    -   Configurations employing separate coils for anchoring and        sensing, wherein the anchoring coil may also serve as an antenna        as shown in any of FIGS. 16B and 18A

Systems and methods employing any of the above listed wireless sensorimplants or resilient sensor constructs may further include any of thefollowing antennas and/or deployment systems:

-   -   Antennas        -   Belt antenna systems            -   Single coil switched between transmit and receive                -   Diode switching            -   Stretchable belt containing constant length antenna wire            -   Orientation of axis of antenna coil aligned with or                parallel to axis of sensor coil        -   Planar antenna systems            -   Separate transmit and receive coils            -   Decoupling of transmit and receive coils to avoid                interference                -   Geometric decoupling    -   Deployment        -   Delivery catheter            -   Delivery sheath            -   Pusher element within sheath            -   Gradual deployment of implant so as to partially contact                lumen wall while partially contained within sheath            -   Retraction of partially deployed implant so as to permit                relocation

Turning to specific alternative RC-WVM implant embodiments disclosedherein, a first exemplary alternative embodiment is RC-WVM implant 12 s,shown in FIGS. 34A, 34B, 34C, and alternative anchor 48 s shown in FIGS.37A, 37B and 37C.

RC-WVM implant 12 s utilizes PTFE coated gold Litz wire 42 s wound onnitinol wire frame 44 s. PTFE has good heat resistance to withstandmanufacturing processes while also being biocompatible. The overallconfiguration of implant 12 s includes strut sections 38 and crownsections 40 substantially as described above. Alternatively, anchors 48s are secured adjacent crown sections 40 as described below. Sections ofheat shrink tubing 61 s are used to help ensure compression of reflowmaterial and may be removed in a later assembly step. A section ofheat-shrink tubing 60 s may be used to cover and insulate capacitor 46s, which in one embodiment may be a 47 nF capacitor, or heat shrinktubing also may be removed as mentioned above.

Capacitor 46 s may be comprised of any suitable structure to provide thedesired capacitance, in one embodiment 47 nF, as mentioned. For example,the desired capacitance may be achieved with a specifically sized gap,different terminal materials (e.g., leads, etc.), overlapping wires, orit could be a gap in a tube with a certain dielectric value. In anexemplary embodiment as illustrated, surface mount capacitor 46 s issoldered between the two terminals 56 s, formed through the joining ofthe 300 strands of Litz wire 42 s. Other electrical attachments such ascrimped, or attached directly to the terminals of the cap brazed with nosolder may also be employed. The capacitor section is then encapsulatedusing a reflow process comprising positioning polymer reflow tube 59 sover the capacitor, connection and terminals, followed by heat-shrinktubing 60 s positioned over the reflow tube. Reflow tube 59 s and heatshrink tube 60 s are placed over the Litz wire/nitinol frame assemblybefore the capacitor before the capacitor is soldered in place (FIG. 35illustrates the reflow and heat shrink tubes for the anchor, which aresimilarly positioned). The tolerances on the O.D.s of these tubes andtheir fit is selected to facilitate assembly, minimize overall profileof the final implant configuration, and optimize the flow of thematerial to increase bond strength. Heat is then applied to melt thepolymer tube and shrink the heat-shrink, thus compressing the moltenpolymer over the capacitor forming a seal. The heat shrink tube is thenremoved. Alternative designs may employ over-moulding processes, adipping process, epoxy potting or similar processes using appropriatebiocompatible materials.

Details of alternative anchors 48 s are shown in FIGS. 37A-C. Anchors 48s are generally formed with at least two sections, an attachment section49 s where the anchor is fixed to the implant and an anchor section 51s, which provides fixation to the vessel wall. In some embodiments, asshown in FIGS. 37A-C, an additional isolation section 53 s is interposedbetween the anchor and attachment sections to allow independentmechanical motion between the anchor section and the attachment sectionin order to help isolate effects of the anchors acting on the vesselwall from the sensing function of the implant. Multiple anchors 48 s maybe used for an anchor system, wherein plural attachment sections 49 sform an anchor system attachment section and plural anchors or anchorsections 48 s form an anchor system anchor section.

Anchor 48 s may be formed by laser cutting a pattern from a nitinol tubeand shape setting the anchor barbs via a heat treatment process. Otherembodiments can be formed using wire of various materials, shape set orbent using a standard process, or laser cut from other metals orbioabsorbable polymers. External surfaces of anchors may utilizedifferent shapes of anchors or different surface finishes to engage thevessel wall and prevent migration of the implant. The overall length ofanchors 48 s that extend beyond crowns sections 40 of implant 12 s isselected to facilitate the expansion of the implant upon deployment fromdelivery system 122 (FIG. 9B) while minimizing the impact of themovement of the implant with the motion of the vessel. This occurs asdescribed above when the distal end of the implant is partially ejectedout of outer sheath 124 and engages with the vessel wall. Length ofanchor protrusion is selected to allow the expansion to effectivelyoccur. If the protrusion is too long, the implant may not deploy in anexpanding, flowering manner as desired. In one embodiment the protrusionof the anchor beyond crown sections 40 (dimension D in FIG. 34B) is lessthan the inner diameter of outer sheath 124 of the delivery catheter.

Attachment section 49 s may be formed using a tube laser cutting processto produce a spiral section of a tube. As indicated in FIG. 35, eachanchor 48 s is positioned by winding the spiral of the attachmentsection around the sensor strut. In one embodiment, the internaldimension of the spiral portion of the attachment section is less thanthe outer dimension of the implant strut 38 so that an interference fitis formed, thus securing the anchor in position. In another embodiment,the internal dimension of the spiral portion is less than the outerdimension of terminal 56 s, but greater than the outer dimension ofimplant strut 38 and can therefore be moved once wrapped into positionon the strut. In one illustrative example, with an implant coil struthaving a nominal diameter of approximately 1.143 mm, the inner diameterof the attachment section spiral may be about 1.156±0.05 mm (with anouter diameter of about 1.556±0.05 mm). In general, relative dimensionsof the implant coil O.D. and anchor spiral I.D. may be selected so as toprovide a locational interference fit.

After placement of the anchor on the implant strut, polymeric reflowtube 59 s is positioned over this assembly and further heat shrink tube61 s placed over this. Heat is then applied to melt the polymer tube andshrink the heat shrink tube, thus forcing the polymer between spacing inthe spiral of the anchor section and thereby reinforcing the fixation ofthe anchor to the implant assembly. Reflow tube 59 s also may be sizedwith a slight interference fit between the outer surface of the implantassembly and the inner surface of the anchor attachment section toprovide some fixation, both longitudinal and rotational, duringassembly. The spacing between the spirals is designed to allow thereflow material to flow into the spaces and form a bond. The width ofthe spirals is designed to allow the spiral section to be manipulatedinto position during assembly, while still providing sufficient rigiditywhen fully assembled. The thickness of the section is minimized toreduce the overall profile of the implant. One advantage of attachmentsection 49 s employing a spiral portion as means of attachment is thatit permits attachment of the anchor to any wire-based implant, includinginsulated wire implants without disturbing or penetrating the insulationlayer. The spiral portion as described distributes the attachment forceacross space of the insulation layer to avoid compromise of the layerand the spaces between the spiral facilitate bonding attachment. Anotheradvantage of attachment using a spiral portion as described is that theaspect ratio of the spiral section may be selected so as to allow thespiral to be slightly unwound to permit placement of the anchor in themiddle of the implant strut section without needing to thread it overthe end past the capacitor terminals. Alternative embodiments ofattachment section 49 s may employ other shapes, such as a T-shaperather than the spiral section, to prevent rotation and detachment fromthe sensor. Further alternatives may also include the replacement ofpolymer reflow tube 59 s with just heat-shrink that could be left inplace, or use an adhesive or other bonding technology.

As shown in FIGS. 37A-C, anchor section 51 s comprises two, laser cutand shape set anchor barbs 50 s. The barbs 50 s are positioned on thevessel facing surface of the anchor and are angled in some embodimentsat between about 10 and 80 degrees to provide fixation with the vesselwall, resistance to cranial and caudal implant migration and to alsofacilitate collapse for loading and deployment of the implant throughits delivery system. Barbs 50 s are shaped to point to engage with thevessel wall and have a length sufficient to penetrate into the vesselwithout perforating through it, typically between about 0.5 and 2.0 mm.The distal end of anchor section 51 s may have a flat end surface 47 sto engage with the pusher of the deployment system and may be filletedto avoid any sharp edges that may cause unnecessary vascular response orcatch on the delivery system. Other alternative embodiments may includemultiple barbs or different surface treatments or barb shapes tooptimize vessel fixation.

Isolation section 53 s is designed to isolate or reduce transmission ofmechanical motion of anchor section 51 s from or to attachment section49 s and thus to the implant, to allow the implant to move freely and atleast substantially free of distortions resulting from contact of theanchor section with the vessel wall. Isolation section 53 s thus maycomprise a narrow cross-section area to provide flexibility whilekeeping thickness constant to provide adequate support. Fillets/curvessurfaces as shown are maintained to avoid stress concentrations thatcould lead to fatigue or unwanted tissue damage. Alternative embodimentsof isolation section 53 s may include varying tube thickness to providemore flexibility or varying the cross-section in a non-mirrored fashionto provide preferential flexibility in one direction.

FIGS. 38A and 38B show alternative embodiments for antenna belt module16 s. In order to accommodate patients of different girth, belt antenna16 s employs a loop antenna wire 82 s mounted on or within base layer 76s, which is wrapped around the patient to form a non-continuouscircumferential loop. Communications link 24 s is provided substantiallyas described above. By using a loop core wire, the core wire forms aloop antenna without having to extend all the way around the patient. Inthis manner, the buckle or clasp (not shown) that closes the belt neednot also provide electrical connections to complete the antenna loop. Asimplified clasp may therefore use a variable connection method such asVelcro or other connection means, thus removing the need for multiplesize belts. As shown in FIGS. 38A and 38B, antenna belt 16 s utilizes asingle (or multiple) loop core wire 82 s wrapped around the patient.Loop ends 83 s of core wire 82 s should be substantially adjacent whenthe base layer is wrapped around the patient, typically within about 2cm to about 10 cm apart. Depending on specific design parameters thesignal strength provided by discontinuous looped core wire 82 s may beless than provided by continuous circumferential core wire 82 asdescribed above. However, depending on application and specific clinicalrequirements, the simplified clasp and ease of use offered by antennabelt module 16 s may offer usability advantages that outweigh the signalrequirements.

Implant repositionability or even recapture with the deployment systemcan be facilitated through the addition of recapture features in thedistal end of the anchor and the pusher tip, exemplary embodiments ofwhich are shown in FIGS. 39A and 39B. Such recapture features allow thesensor to remain attached to the pusher while being partially deployed.From this point the sensor can be fully deployed using the mechanism,the device repositioned as the sensor is still attached to the pusher,or recaptured by advancing the sheath over the sensor and the removal ofthe sensor. These features can take many forms including interlockingelements, screws, or release bumps. In one embodiment, as illustrated inFIG. 39A, recapture features 127, 129 may include a “T shaped” extension127 to the anchor, which engages with an appropriately shaped recess 129in the distal end of pusher 126. In another alternative, shown in FIG.39B, recapture features 127′, 129′ include through-hole 127′ in thedistal end of the anchor through which pin-shaped extension 129′ frompusher 126 engages to provide engagement while retained within outersheath 124. Such recapture features could be used to partially deploythe sensor, while retaining the ability to reposition or recapture it.The recapture features remain engaged while the distal end of the anchorremains within the sheath. When the operator is satisfied with the finalposition, the sheath would be withdrawn fully, thus releasing theinterlocking features and deploying the sensor.

While anchors 48 s are shown in FIGS. 34A-C as attached only at one endof the implant (to facilitate flowering deployment as described), it iscontemplated that anchors may be placed at both ends of an implant, withfewer or more anchors provided as compared to the four shown in thefigures.

In other alternative embodiments, as illustrated in FIGS. 40-52D, one ormore anchor elements to help prevent migration may be provided as anintegrated anchor frame, as opposed to individual anchor elementsdescribed hereinabove. In one example, as shown in FIG. 40, an RC-WVMimplant comprises anchor frame 650 is attached to RC-WVM sensor section12 t. The RC-WVM sensor section (or just “sensor section”) 12 t maycomprise any previously described “Z-shaped” coils or similar RC-WVMimplant 12 as described above generally comprising strut sections 38joined by crown sections 40. For the sake of clarity, hereinafter, withrespect to embodiments described in reference to FIGS. 40-52D, RC-WVMimplant (or “implant” alone) refers to the combined RC-WVM sensorsection and anchor frame 650. Anchor frame 650 may be formed of nitinolwire or laser cut tubing whereby the tube is expanded to the equivalentdiameter of the sensor section. Nitinol, or other materials with similarproperties, is well-suited as material for anchor frame 650 because itallows the anchor frame to collapse to the same loaded configuration inthe loader as the RC-WVM sensor section (see FIG. 9D.)

FIG. 41 shows an example of anchor frame 650 before it is attached to asensor section, such as RC-WVM sensor section 12 t. Similar to RC-WVMsensor section 12 t, anchor frame 650 comprises a series of straightstrut sections 652 (also referred to as anchor sections) joined bycurved crown sections 654 to form a resilient, concentric zig-zag orlinked “Z-shapes” structure, which may also be considered to besinusoidal in appearance. One or more anchor barbs 656 are disposedwithin the strut sections or anchor sections as described in more detailbelow. Anchor frame 650 as shown in FIGS. 40 and 41 includes only asingle anchor barb 656 on each strut section 652. Anchor frame 650 isattached to the sensor section by attachment arms 658 that overlap strutsections 652 of the sensor section. Note also that crown sections 654 onthe end opposite attachment sections may be provided with recapturefeatures such as recapture features 127, 127′, as shown in FIGS. 39A and39B, which mate with corresponding recapture features 129, 129′ formedon the distal end of deployment pusher 126.

As best seen in detail in FIG. 42, polymeric reflow tube 660 ispositioned over attachment arm 658 and further heat shrink tube 662placed over the reflow tube. As illustrated in FIG. 42, attachment arm658 is visible through transparent reflow and heat shrink tubes 660 and662. Heat is then applied to melt polymer reflow tube 660 and shrink theheat shrink tube 662, thus forcing the polymer between and aroundattachment arm 658 and thereby fixing anchor frame 650 to the RC-WMVsensor section. Reflow tube 660 may be sized with a slight interferencefit between the outer surface of strut section 38 and an inner surfaceof the reflow tube to provide some stability, both longitudinal androtational, during assembly. Attachment arms 658 may be configured toinclude an anchor isolation section 659. Isolation section 659 is oneform isolation means as previously described. Radial force requirementsof anchor frame 650 and the function of isolation section 659 are alsodiscussed in more detail below.

Attachment arm 658 may contain a saw tooth-like configuration as shownin FIG. 42 wherein spaces between teeth 664 allow the reflow material toflow in between and form a more secure bond. Other, alternativeconfigurations for attachment arms 658, which provide this increasedsurface are considered to increase the bond strength such as zig zags,T-connectors, S connectors, and voids in center of struts are shown,respectively, in FIGS. 52A-D. Further alternatives include surfacefinishes or texturing on attachment arms 658. In certain designs suchalternative configurations may permit the thickness of the attachmentarm to be minimized to reduce the overall profile of the implant.

In some embodiments, for example as shown in FIGS. 41 and 43, it may bedesirable to provide split 666 in anchor frame 650 so as to not producea continuous ring of conductive material that could cause interferencewith sensor readings. Split 666 provides a break in the anchor frame toprevent the magnetic field from the external reader coupling into theanchor frame and potentially providing interference from the RC-WVMimplant signal generated by the sensor section. Split 666 in anchorframe 650 advantageously is located at close to the sensor section, forexample approximately at the center of an anchor frame crown 654 so thatthe split in the frame does not significantly compromise structuralintegrity of anchor frame 650. In one such example, as shown in FIGS. 41and 43, split crown 654S is provided with double attachment arms 658,one securable to each strut section 38 on opposite sides ofcorresponding implant crown section 654. In other embodiments, the splitmay be located elsewhere on the anchor frame as further described below.If desired, double attachment arms 658 may be provided for non-splitanchor crowns 654 as well.

In other embodiments, the decoupling split 666 of the anchor frame maybe located elsewhere on the frame and, in such cases, preferablystructurally reinforced by bridging with an additional metallic orpolymeric component that provides sufficient structural integrity to theanchor frame while maintaining the discontinuous configuration.Alternatively, a continuous anchor frame structure may be devised bycarefully selecting the amount of metallic material of the frame andshape of the frame to minimize or control interference with the RC-WVMimplant signal such that it may be otherwise compensated for in signalprocessing.

In some embodiments, anchor frame 650 may be attached to the RC-WVMsensor section and loaded in the deployment system with the orientationof the anchor frame exposed first during deployment. In this case,pusher 126 of delivery system 122 bears on crown sections 40 of thesensor section (see, e.g., FIG. 9D). In other embodiments thisconfiguration may be reversed, with the sensor section deployed firstand the pusher of the deployment system bearing on crowns 654 of anchorframe 650. The orientation may be varied depending on factors such asthe access site for implantation, e.g. femoral vein versus jugular vein.In a further alternative, as shown in FIG. 44, for increased anchoringan anchor frame 650 may be provided on each end of the RC-WVM implant(such as sensor section 12 t), in which case the anchor frame would befirst deployed regardless of orientation of the RC-WVM implant in thedelivery system.

Once an RC-WVM implant employing anchor frame 650 is deployed within avessel, barbs 656 engage with the vessel wall in various orientations toprevent movement of the device. FIGS. 45A, 45B and 45C show oneembodiment of anchor frame 650 a in which anchor barbs 656 a are setparallel with anchor frame struts 652. Note also that anchor frame 650may employ two attachment arms 658 at each implant facing crown, whereinsome arms are provided with saw teeth 664 and some without. In anotherembodiment, the plane of the anchor barb direction can be offset suchthat it is in the axial direction of the flow of the blood within theIVC or any increment in between corresponding to axial direction overthe indicated sizing range for the RC-WVM implant. FIG. 45C depicts ananchor barb 656 a which in its final shape state lies parallel to thestrut 650 a which it is attached to, but is shape set such that itspointed tip is out of plane defined through the strut and parallel barb,that is out of the plane of the page as shown in FIG. 45C. This out ofplane protrusion facilitates the anchor engaging with the vessel wall,preventing migration. The deployed configuration of this anchor is shownin FIG. 45A, with the anchor parallel to the strut 650 a and thereforeat an angle to the direction of blood flow in the vessel.

In another example, as shown in FIGS. 46A, 46B and 46C, axially facinganchor barbs 656 b are positioned such that when anchor frame 650 b isdeployed within a vessel, anchor barbs 656 b run parallel (or close toparallel) to the vessel direction and to the flow within the vessel. Ina further embodiment, shown in FIGS. 47A and 47B, anchor barbs 656 c ofanchor frame 650 c are located at crowns 654 of the anchor frame andshape set outwardly so as to engage the vessel wall. FIGS. 47A and 47Balso provide an example of possible, approximate dimensions for anembodiment of an anchor frame. FIG. 46C depicts an anchor barb 656 bwhich in its final shape state lies at an angle to the strut 650 b whichit is attached to, and is shape set such that its pointed tip is alsoout of the plane defined between the anchor barb and the strut to whichit is attached. This out of plane protrusion in two axes, facilitatesthe anchor engaging with the vessel wall in a more optimal, more axialorientation, potentially providing increased migration resistance. Thedeployed configuration of this anchor is shown in FIG. 46A, with theanchor at an angle to the strut 650 b and therefore generally parallelto direction of blood flow in the vessel. This final position of theanchor tip, out of plane from the strut in two axes can also be seen inFIG. 48A.

FIG. 48A depicts an anchor frame embodiment 650 a, which is formed withstraight strut sections 652 s between crown sections 654. Straight strutsections 652 s can provide an advantage of the strut section alwaysbeing in contact with the vessel wall over its entire length,irrespective of the size of vessel into which it is deployed. When theframe is formed, for example, by laser cutting the construct from anitinol tube, the straight configuration of straight strut sections 652s can be achieved by shape-setting the strut sections to maintain thedesired straight configuration. FIG. 48B shows an alternative anchorframe embodiment 650 b, which is formed around the surface of acylindrical shape setting mandrel resulting in curved strut sections 652c. Curved strut sections 652 c can provide the advantage of increasingthe local force urging anchor barbs 656 (shown as double barbs) into thevessel wall for fixation, but may be associated with a disadvantage ofthe crowns not being in contact with the vessel wall, especially whenthe device is implanted in a small vessel.

Various orientations and configurations of anchor barbs 656 may beprovided in different embodiments as illustrated in FIGS. 49A-49G. Forexample, as shown in FIG. 49A, anchor barb 656 may extend outwardly atthe center of each strut 652 of anchor frame 650 at an angle (A) betweenabout 10° and 90°. Anchor barbs 656 may alternately face in either orboth the caudal or cranial direction in the plane of the shape set strut652 or extend out of that plane. In another embodiment, as shown in FIG.49B, there may be multiple anchor barbs 656 a on each strut 652 facingeach direction. Multiple anchor barbs 656 a as shown in FIG. 49B arelocated on one side of strut 652 facing in opposite directions, whereasin FIG. 49E, anchor barbs are on opposite sides of the strut, facing inthe same direction. In another embodiment, shown in FIGS. 49C and 49D,anchor barbs 656 b are contained within the thickness of strut 652, asopposed to being located on the side of the strut as shown, for example,in FIGS. 49A and 49B. The anchor barb configuration shown in FIGS. 49C-Dmay be formed in a similar manner to anchor barbs 50 s as shown in FIGS.37A-C and described above.

In other embodiments, examples of which are shown in FIGS. 49E-H, anchorbarbs 656 may have overall shapes and/or points of differentconfigurations, which may aid insertion and retention of the anchor barbwithin the vessel wall in various clinical situations. FIG. 49Eillustrates single pronged barb 656 c and fish hook barb 656 dpositioned on opposite sides of strut 652, facing in the same direction.FIGS. 49F, 49G and 49H show further examples of anchor barb designs, inthis case saw-teeth barb 656 e, double edged barb 656 f, and doublesided, hooked barb 656 g, respectively. These barbs also can be locatedon the side of the anchor frame strut and also within the thickness ofthe strut as previously described

As described above, it may be desirable to configure anchor frame 650 sothat it does not form a coil that could interfere with the RC-WVMimplant signal. One solution, as described above is split 666. In otherembodiments, for example where other design considerations may make adiscontinuous structure less preferable such that anchor frame wire ismechanically and electrically joined (e.g. a crimped joint), theterminations of the wire ends where joined and in contact with eachother may be electrically insulated so as to not form coil capable ofcoupling with a magnetic field. An example of such insulation is apolymer coating. In other embodiments, for example, where the anchorframe may be formed of nitinol laser cut tubing, for which a mechanicaljoint or bond may be required, the terminations of the nitinol frame canbe physically and electrically separated by use of a non-conductingbonding agent such as a polymer, epoxy or ceramic material. FIG. 50illustrates such a non-conducting joint in cross-section. In thisexample, ends 670 of anchor frame 650 have interlocking portions whichmay be bonded with non-conducting bonding agent 672, which alsosurrounds the joint for increased strength.

As previously discussed, the radial force exerted by the RC-WVM implantshould be such that the sensor section moves with the natural motion ofthe IVC as it expands and contracts due to changes in fluid volume.Anchor frame 650 is configured to exert an outward radial force that issufficient to ensure engagement of anchor barbs 656 into the vessel wallto help prevent migration along the vessel without interference withmotion and electrical performance of the RC-WVM sensor section. Thus,the radial force exerted by anchor frame 650 typically may be equal toor higher than that exerted by the sensor section of the RC-WVM implant,so as to provide migration resistance while substantially isolated byisolation section 659 from the lower radial force sensor section, which,is configured to permit natural expansion and contraction of the IVC inresponse to varying fluid status.

Isolation section 659 allows attachment between the sensor section andanchor frame, but also permits the sensor section and anchor frame toact independently of each other. Thus, the RC-WVM sensor section cancontract and expand at the monitoring location within the vesselindependently of anchor frame expansion and contraction at the anchoringlocation in the vessel. One design consideration in selecting theconfiguration of the anchor frame is that the radial force exerted bythe anchor frame should be sufficient to prevent migration of the RC-WVMimplant, but low enough so as to not stent or prop open the vessel.

FIG. 51 illustrates one example of how the radial force of anchor frame650 can be adjusted or modified to control the radial force exerted byaltering the configuration, via changes in shape set diameter, strutwidth, strut thickness, strut shape, crown diameter, number of crowns,strut length, material properties, distance between the sensor sectionand anchor frame, and overall length. Another alternative to increasethe fixation of the RC-WVM implant is to provide anchor frames on bothends of the sensor section, as shown in the example of FIG. 44. FIG. 51shows an alternative anchor frame 650 a with relatively short strut 652lengths, more crowns 654 (here 16 crowns instead of 8 as in earlierembodiments), and smaller crown diameters. Isolation sections 659 arealso longer so that the distance between the anchor frame and sensorsection is increased.

The configuration of anchor frame 650 a in FIG. 51 is selected forappropriate radial force while minimizing areas of high strainconcentration that could lead to reduced fatigue life. Factors thataffect the amount of radial force that can be exerted by the anchorframe without undue effect on the sensor section include the distancebetween anchor barbs 656 and the sensor section, which can be adjustedbased on the position of the anchor barbs on strut 652 and/or by thelength of isolation section 659 that also assists with isolation. Inaddition to varying the length of isolation section 659, otheradjustments include varying the thickness and/or straight versus curvedsections. For example, a straight anchor isolation section 659 is shownin FIG. 51, and in another example, a curved or s-shaped anchorisolation section 659 is shown FIG. 47A.

In another alternative embodiment, the anchor frame may be configured soas to intentionally fracture and self-separate from the sensor sectionover time. In this embodiment, connection points between the anchorframe and sensor section, for example in isolation section 659, aredesigned to deliberately fracture. The purpose of the deliberatefracture is to completely isolate the anchor frame from the sensorsection after fracture. In such an embodiment, the anchor frame wouldsecure the RC-WVM implant against migration when first deployed in thevessel. Over time, as the sensor section embeds into the tissue, therisk of migration diminishes. As a result, the anchor frame's functionis no longer required. This embodiment allows for disconnection of theanchor frame from the device once it is no longer required without theneed for surgical intervention.

The material and design of the isolation sections 659 may be selected toprovide for different time periods for fracture to occur. For example,the geometry, design, movement and material of the sensor section,isolation section and anchor frame can be tuned for a fatigue inducedfracture to occur after/within a given time due to fatigue.Alternatively, fracture can be induced by external means. For exampleultra sound/RF may be used to induce fracture by breaking down thematerial or bond between the anchor frame and sensor section at apre-set frequency or energy. In a further alternative embodiment,chemically induced fracture of isolation sections 659 may be achievedwith, for example, a biodegradable polymer such as PLA, PCL, PLGA, PLGor other as the bond/connection between the anchor frame and RC-WVMimplant frame. Chemically induced fracture takes advantage of thematerial properties of biodegradable polymers, which can degrade atcontrolled rates including such as of pH, temperature, microorganismspresent, and water etc.

In another alternative embodiment, anchor frame 650 may be made of abioabsorbable/biodegradable material such as commonly used forbioabsorbable stents. Similar to other embodiments of the anchor frame,the purpose of a bioabsorbable anchor frame is to help preventmigration. Once again, as the sensor section embeds into the tissue overtime, the risk of migration diminishes. As a result, the anchor frame'sfunction is no longer required. The material and design of abioabsorbable anchor frame may be selected for different time periodsfor absorption.

The foregoing has been a detailed description of illustrativeembodiments of the invention. It is noted that in the presentspecification and claims appended hereto, conjunctive language such asis used in the phrases “at least one of X, Y and Z” and “one or more ofX, Y, and Z,” unless specifically stated or indicated otherwise, shallbe taken to mean that each item in the conjunctive list can be presentin any number exclusive of every other item in the list or in any numberin combination with any or all other item(s) in the conjunctive list,each of which may also be present in any number. Applying this generalrule, the conjunctive phrases in the foregoing examples in which theconjunctive list consists of X, Y, and Z shall each encompass: one ormore of X; one or more of Y; one or more of Z; one or more of X and oneor more of Y; one or more of Y and one or more of Z; one or more of Xand one or more of Z; and one or more of X, one or more of Y and one ormore of Z.

Various modifications and additions can be made without departing fromthe spirit and scope of this invention. Features of each of the variousembodiments described above may be combined with features of otherdescribed embodiments as appropriate in order to provide a multiplicityof feature combinations in associated new embodiments. Furthermore,while the foregoing describes a number of separate embodiments, what hasbeen described herein is merely illustrative of the application of theprinciples of the present invention. Additionally, although particularmethods herein may be illustrated and/or described as being performed ina specific order, the ordering is highly variable within ordinary skillto achieve aspects of the present disclosure. Accordingly, thisdescription is meant to be taken only by way of example, and not tootherwise limit the scope of this invention.

Exemplary embodiments have been disclosed above and illustrated in theaccompanying drawings. It will be understood by those skilled in the artthat various changes, omissions and additions may be made to that whichis specifically disclosed herein without departing from the spirit andscope of the present invention.

What is claimed is:
 1. An implantable vessel monitoring deviceconfigured and dimensioned to be implanted in a patient blood vessel incontact with the vessel wall, said device comprising: an expandable andcollapsible variable inductance coil comprising a plurality of adjacentwire strands formed around an open center to allow substantiallyunimpeded blood flow therethrough, said coil configured and dimensioned(i) to extend around an inner periphery of the vessel when implantedtherein and (ii) to move with the vessel wall in response to expansionand collapse of the vessel; and a capacitance which together with saidvariable inductance coil forms a variable inductance resonant circuithaving a variable characteristic frequency correlated to the diameter orarea of the expandable and collapsible variable inductance coil.
 2. Theimplantable vessel monitoring device of claim 1, wherein said expandableand collapsible variable inductance coil comprises more than 20 wirestrands.
 3. The implantable vessel monitoring device of claim 2, whereinsaid expandable and collapsible variable inductance coil comprisesapproximately 300 substantially parallel wire strands formed in a singleloop around the open center.
 4. The implantable vessel monitoring deviceof claim 1, wherein said expandable and collapsible variable inductancecoil comprises about 10-20 wire strands formed in about 15-25 loopsaround the open center.
 5. The implantable vessel monitoring device ofclaim 1, wherein the capacitance comprises a capacitor in electricalcommunication with said variable inductance coil.
 6. The implantablevessel monitoring device of claim 1, wherein said capacitance comprisesan inherent capacitance produced by spaces between said adjacent wirestrands.
 7. The implantable vessel monitoring device of claim 1, whereinsaid plurality of wire strands are each individually insulated.
 8. Theimplantable vessel monitoring device of claim 1, wherein said pluralityof wire strands are covered by an outer layer.
 9. The implantable vesselmonitoring device of claim 1, wherein: said coil is formed from a singleloop of said plurality of wire strands. wherein the plurality of wirestrands is wrapped in a rope-like manner around a central core.
 10. Theimplantable vessel monitoring device of claim 1, wherein said expandableand collapsible variable inductance coil comprises multiple strands ofwire laid parallel to each other and twisted into bundles, with thebundles wrapped multiple times around the open center.
 11. Theimplantable vessel monitoring device of claim 1, further comprising aflexible sensor frame formed in an open center loop supporting saidexpandable and collapsible variable inductance coil.
 12. The implantablevessel monitoring device of claim 11, wherein said plurality of wirestrands completely cover the resilient sensor frame.
 13. The implantablevessel monitoring device of claim 11, wherein said flexible sensor framecomprises an elongate flexible member having a longitudinal dimensionand a transverse dimension, and said plurality of wire strands arewrapped around the transverse dimension of the elongate flexible member.14. The implantable vessel monitoring device of claim 11, wherein: saiddevice is configured and dimensioned to be implanted in the vena cava;and the flexible sensor frame is configured and dimensioned to form aloop surrounding an inner periphery of the vena cava when implantedtherein, said loop being radially collapsible and expandable whenimplanted and having a combination of flexibility and stiffness selectedto (i) support and maintain said variable inductance coil in contactwith the vena cava wall, (ii) allow the vena cava to collapse and expandover the respiratory and cardiac cycles, and (iii) allow the variableinductance coil supported by said frame to change shape as a magnitudeof collapse and expansion of the vena cava varies with vascular fluidvolume of the patient.
 15. The implantable vessel monitoring device ofclaim 11, wherein said flexible sensor frame is formed by a singleflexible frame member forming a single loop.
 16. The implantable vesselmonitoring device of claim 15, wherein said flexible frame memberincludes a capacitor or gap disposed in said loop.
 17. The implantablevessel monitoring device of claim 11, further comprising a device anchorattached to said flexible sensor frame.
 18. The implantable vesselmonitoring device of claim 17, wherein the device anchor comprises aplurality of individual anchor members extending from said flexiblesensor frame.
 19. The implantable vessel monitoring device of claim 17,wherein the device anchor comprises a separate anchor frame secured tothe flexible sensor frame, the separate anchor frame being formed in anexpandable and collapsible loop.
 20. The implantable vessel monitoringdevice of claim 17, wherein said device anchor comprises: pluralattachment sections attached to the flexible sensor frame at spacedapart locations on the flexible sensor frame, the attachment sectionsdefining spaces between structure of the attachment section for abonding agent to enter and attach to the flexible sensor frame; and atleast one anchor section connected to each said attachment section withat least one tissue-engaging anchor barb disposed in each said anchorsection.
 21. The implantable vessel monitoring device of claim 20,wherein said device anchor further comprises an anchor isolation sectiondisposed between the attachment section and at least one anchor section,the anchor isolation section being configured to allow relative motionbetween the anchor section and attachment section.
 22. The implantablevessel monitoring device of claim 21, wherein said device anchor furthercomprises plural anchor sections joined by crown sections to form ananchor frame with a resilient, concentric zig-zag structure.
 23. Theimplantable vessel monitoring device of claim 22, wherein said anchorsections and crown sections are formed in a loop of resilient conductivematerial with an electrical gap therein.
 24. An implantable vesselmonitoring device configured and dimensioned to be implanted in apatient blood vessel in contact with the blood vessel wall, said devicecomprising an expandable and collapsible variable inductance coilcomprising at least about 150 adjacent wire strands formed around anopen center, said coil configured and dimensioned (i) to extend aroundan inner periphery of the blood vessel when implanted therein and (ii)to move with the blood vessel wall in response to expansion and collapseof the blood vessel; a sensor frame formed in a single loop having aloop circumference on which said variable inductance coil is supported,the plurality of wire strands forming a wire coating over said sensorframe and being wrapped in at least one loop around said loopcircumference of the sensor frame; and a capacitance which together withsaid variable inductance coil forms a variable inductance resonantcircuit having a variable characteristic frequency correlated to thediameter or area of the expandable and collapsible variable inductancecoil.
 25. The implantable vessel monitoring device of claim 24, furthercomprising a device anchor, wherein the device anchor comprises: pluralattachment sections attached to said sensor frame at spaced apartlocations on said sensor frame; at least one anchor section connected toeach said attachment section with at least one tissue-engaging anchorbarb disposed in each said anchor section; and an anchor isolationsection disposed between each attachment section and anchor section, theanchor isolation section being configured to allow relative motionbetween the anchor section and attachment section.
 26. The implantablevessel monitoring device of claim 25, wherein: said device is configuredand dimensioned to be implanted in the vena cava; and the sensor frameis configured and dimensioned to extend around an inner periphery of thevena cava when implanted therein, said loop being radially collapsibleand expandable when implanted and having a combination of flexibilityand stiffness selected to (i) support and maintain said variableinductance coil in contact with the vena cava wall, (ii) allow the venacava to collapse and expand over the respiratory and cardiac cycles, and(iii) allow the variable inductance coil supported by said sensor frameto change shape as a magnitude of collapse and expansion of the venacava varies with vascular fluid volume of the patient.
 27. Animplantable vessel monitoring device configured and dimensioned to beimplanted in a patient vena cava in contact with the vena cava wall,said device comprising: an expandable and collapsible variableinductance coil comprising a plurality of substantially parallel wirestrands formed around an open center, said coil configured anddimensioned (i) to surround an inner periphery of the vena cava whenimplanted therein and (ii) to move with the vena cava wall in responseto changes in fluid volume or movement of the vena cava wall over therespiratory and cardiac cycles; a flexible sensor frame supporting saidvariable inductance coil and being configured and dimensioned (i) tomaintain the variable inductance coil in contact with the vena cavawall, (ii) to move with expansion and collapse of the vena cava wallover the respiratory and cardiac cycles, and (iii) to allow changes inshape of the variable inductance coil corresponding to a magnitude ofcollapse and expansion as the vena cava varies with changes in vascularfluid volume of the patient; a capacitance which together with saidvariable inductance coil forms a variable inductance resonant circuithaving a variable characteristic frequency correlated to the diameter orarea of the expandable and collapsible variable inductance coil; and atleast one device anchor attached to said resilient loop frame by atleast one anchor isolation section, the anchor isolation section beingconfigured to allow relative motion between the at least one deviceanchor and the resilient loop frame.
 28. The implantable vesselmonitoring device of claim 27, wherein said at least one device anchorcomprises an anchor frame having a resilient, concentric zig-zagstructure formed by plural anchor sections joined by crown sections withat least one anchor barb disposed in each said anchor section.
 29. Theimplantable vessel monitoring device of claim 27, wherein said flexiblesensor frame comprises a single resilient member having pluralsubstantially straight sections connected by bends to form a zig-zagshaped flexible sensor frame, and said parallel wire strands are twistedin a bundle, with the bundle wrapped on said sensor frame multiple timesaround the open center.
 30. An implantable vessel monitoring deviceconfigured and dimensioned to be implanted in a patient vena cava incontact with the vena cava wall, said device comprising: an expandableand collapsible variable inductance coil comprising at least about 150adjacent wire strands formed around an open center to allowsubstantially unimpeded blood flow therethrough, said coil configuredand dimensioned (i) to extend around an inner periphery of the vena cavawhen implanted therein and (ii) to move with the vena cava wall inresponse to changes in fluid volume or movement of the vena cava wallover the respiratory and cardiac cycles; a sensor frame formed in asingle loop having a loop circumference with plural substantiallystraight sections connected by bends to form a resilient, open center,zig-zag shaped loop structure supporting said expandable and collapsiblevariable inductance coil with said plurality of wire strands beingwrapped in at least one loop around said loop circumference of thesensor frame to form a wire coating covering the sensor frame; an outerinsulating layer covering the plurality of wires and sensor frame; acapacitance which together with said variable inductance coil forms avariable inductance resonant circuit having a variable characteristicfrequency correlated to the diameter or area of the expandable andcollapsible variable inductance coil; a zig-zag loop anchor frame formedby plural anchor sections joined by crown sections, with at least oneanchor barb disposed in each said anchor section; and at least oneanchor isolation section concentrically joining the zig-zag loop anchorframe with said zig-zag loop sensor frame, the anchor isolation sectionbeing configured to allow relative motion between said sensor frame andsaid anchor frame.